- 09 Oct, 2013 39 commits
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Mel Gorman authored
This patch implements a system-wide search for swap/migration candidates based on total NUMA hinting faults. It has a balance limit, however it doesn't properly consider total node balance. In the old scheme a task selected a preferred node based on the highest number of private faults recorded on the node. In this scheme, the preferred node is based on the total number of faults. If the preferred node for a task changes then task_numa_migrate will search the whole system looking for tasks to swap with that would improve both the overall compute balance and minimise the expected number of remote NUMA hinting faults. Not there is no guarantee that the node the source task is placed on by task_numa_migrate() has any relationship to the newly selected task->numa_preferred_nid due to compute overloading. Signed-off-by:
Mel Gorman <mgorman@suse.de> [ Do not swap with tasks that cannot run on source cpu] Reviewed-by:
Rik van Riel <riel@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> [ Fixed compiler warning on UP. ] Signed-off-by:
Peter Zijlstra <peterz@infradead.org> Link: http://lkml.kernel.org/r/1381141781-10992-40-git-send-email-mgorman@suse.deSigned-off-by:
Ingo Molnar <mingo@kernel.org>
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Peter Zijlstra authored
Use the new stop_two_cpus() to implement migrate_swap(), a function that flips two tasks between their respective cpus. I'm fairly sure there's a less crude way than employing the stop_two_cpus() method, but everything I tried either got horribly fragile and/or complex. So keep it simple for now. The notable detail is how we 'migrate' tasks that aren't runnable anymore. We'll make it appear like we migrated them before they went to sleep. The sole difference is the previous cpu in the wakeup path, so we override this. Signed-off-by:
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Peter Zijlstra authored
Introduce stop_two_cpus() in order to allow controlled swapping of two tasks. It repurposes the stop_machine() state machine but only stops the two cpus which we can do with on-stack structures and avoid machine wide synchronization issues. The ordering of CPUs is important to avoid deadlocks. If unordered then two cpus calling stop_two_cpus on each other simultaneously would attempt to queue in the opposite order on each CPU causing an AB-BA style deadlock. By always having the lowest number CPU doing the queueing of works, we can guarantee that works are always queued in the same order, and deadlocks are avoided. Signed-off-by:
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Mel Gorman authored
Base page PMD faulting is meant to batch handle NUMA hinting faults from PTEs. However, even is no PTE faults would ever be handled within a range the kernel still traps PMD hinting faults. This patch avoids the overhead. Signed-off-by:
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Mel Gorman authored
NUMA hinting faults will not migrate a shared executable page mapped by multiple processes on the grounds that the data is probably in the CPU cache already and the page may just bounce between tasks running on multipl nodes. Even if the migration is avoided, there is still the overhead of trapping the fault, updating the statistics, making scheduler placement decisions based on the information etc. If we are never going to migrate the page, it is overhead for no gain and worse a process may be placed on a sub-optimal node for shared executable pages. This patch avoids trapping faults for shared libraries entirely. Signed-off-by:
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Rik van Riel authored
When a task is already running on its preferred node, increment numa_migrate_seq to indicate that the task is settled if migration is temporarily disabled, and memory should migrate towards it. Signed-off-by:
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Mel Gorman authored
When a preferred node is selected for a tasks there is an attempt to migrate the task to a CPU there. This may fail in which case the task will only migrate if the active load balancer takes action. This may never happen if the conditions are not right. This patch will check at NUMA hinting fault time if another attempt should be made to migrate the task. It will only make an attempt once every five seconds. Signed-off-by:
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Mel Gorman authored
This patch replaces find_idlest_cpu_node with task_numa_find_cpu. find_idlest_cpu_node has two critical limitations. It does not take the scheduling class into account when calculating the load and it is unsuitable for using when comparing loads between NUMA nodes. task_numa_find_cpu uses similar load calculations to wake_affine() when selecting the least loaded CPU within a scheduling domain common to the source and destimation nodes. It avoids causing CPU load imbalances in the machine by refusing to migrate if the relative load on the target CPU is higher than the source CPU. Signed-off-by:
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Mel Gorman authored
There is a 90% regression observed with a large Oracle performance test on a 4 node system. Profiles indicated that the overhead was due to contention on sp_lock when looking up shared memory policies. These policies do not have the appropriate flags to allow them to be automatically balanced so trapping faults on them is pointless. This patch skips VMAs that do not have MPOL_F_MOF set. [riel@redhat.com: Initial patch] Signed-off-by:
Joe Mario <jmario@redhat.com> Reviewed-by:
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Rik van Riel authored
The load balancer can move tasks between nodes and does not take NUMA locality into account. With automatic NUMA balancing this may result in the tasks working set being migrated to the new node. However, as the fault buffer will still store faults from the old node the schduler may decide to reset the preferred node and migrate the task back resulting in more migrations. The ideal would be that the scheduler did not migrate tasks with a heavy memory footprint but this may result nodes being overloaded. We could also discard the fault information on task migration but this would still cause all the tasks working set to be migrated. This patch simply avoids migrating the memory for a short time after a task is migrated. Signed-off-by:
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Mel Gorman authored
Ideally it would be possible to distinguish between NUMA hinting faults that are private to a task and those that are shared. If treated identically there is a risk that shared pages bounce between nodes depending on the order they are referenced by tasks. Ultimately what is desirable is that task private pages remain local to the task while shared pages are interleaved between sharing tasks running on different nodes to give good average performance. This is further complicated by THP as even applications that partition their data may not be partitioning on a huge page boundary. To start with, this patch assumes that multi-threaded or multi-process applications partition their data and that in general the private accesses are more important for cpu->memory locality in the general case. Also, no new infrastructure is required to treat private pages properly but interleaving for shared pages requires additional infrastructure. To detect private accesses the pid of the last accessing task is required but the storage requirements are a high. This patch borrows heavily from Ingo Molnar's patch "numa, mm, sched: Implement last-CPU+PID hash tracking" to encode some bits from the last accessing task in the page flags as well as the node information. Collisions will occur but it is better than just depending on the node information. Node information is then used to determine if a page needs to migrate. The PID information is used to detect private/shared accesses. The preferred NUMA node is selected based on where the maximum number of approximately private faults were measured. Shared faults are not taken into consideration for a few reasons. First, if there are many tasks sharing the page then they'll all move towards the same node. The node will be compute overloaded and then scheduled away later only to bounce back again. Alternatively the shared tasks would just bounce around nodes because the fault information is effectively noise. Either way accounting for shared faults the same as private faults can result in lower performance overall. The second reason is based on a hypothetical workload that has a small number of very important, heavily accessed private pages but a large shared array. The shared array would dominate the number of faults and be selected as a preferred node even though it's the wrong decision. The third reason is that multiple threads in a process will race each other to fault the shared page making the fault information unreliable. Signed-off-by:
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Mel Gorman authored
task_numa_work skips small VMAs. At the time the logic was to reduce the scanning overhead which was considerable. It is a dubious hack at best. It would make much more sense to cache where faults have been observed and only rescan those regions during subsequent PTE scans. Remove this hack as motivation to do it properly in the future. Signed-off-by:
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Mel Gorman authored
Currently automatic NUMA balancing is unable to distinguish between false shared versus private pages except by ignoring pages with an elevated page_mapcount entirely. This avoids shared pages bouncing between the nodes whose task is using them but that is ignored quite a lot of data. This patch kicks away the training wheels in preparation for adding support for identifying shared/private pages is now in place. The ordering is so that the impact of the shared/private detection can be easily measured. Note that the patch does not migrate shared, file-backed within vmas marked VM_EXEC as these are generally shared library pages. Migrating such pages is not beneficial as there is an expectation they are read-shared between caches and iTLB and iCache pressure is generally low. Signed-off-by:
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Mel Gorman authored
task_numa_placement checks current->mm but after buffers for faults have already been uselessly allocated. Move the check earlier. [peterz@infradead.org: Identified the problem] Signed-off-by:
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Mel Gorman authored
Ideally it would be possible to distinguish between NUMA hinting faults that are private to a task and those that are shared. This patch prepares infrastructure for separately accounting shared and private faults by allocating the necessary buffers and passing in relevant information. For now, all faults are treated as private and detection will be introduced later. Signed-off-by:
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Mel Gorman authored
A preferred node is selected based on the node the most NUMA hinting faults was incurred on. There is no guarantee that the task is running on that node at the time so this patch rescheules the task to run on the most idle CPU of the selected node when selected. This avoids waiting for the balancer to make a decision. Signed-off-by:
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Mel Gorman authored
Just as "sched: Favour moving tasks towards the preferred node" favours moving tasks towards nodes with a higher number of recorded NUMA hinting faults, this patch resists moving tasks towards nodes with lower faults. Signed-off-by:
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Mel Gorman authored
This patch favours moving tasks towards NUMA node that recorded a higher number of NUMA faults during active load balancing. Ideally this is self-reinforcing as the longer the task runs on that node, the more faults it should incur causing task_numa_placement to keep the task running on that node. In reality a big weakness is that the nodes CPUs can be overloaded and it would be more efficient to queue tasks on an idle node and migrate to the new node. This would require additional smarts in the balancer so for now the balancer will simply prefer to place the task on the preferred node for a PTE scans which is controlled by the numa_balancing_settle_count sysctl. Once the settle_count number of scans has complete the schedule is free to place the task on an alternative node if the load is imbalanced. [srikar@linux.vnet.ibm.com: Fixed statistics] Signed-off-by:
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Mel Gorman authored
NUMA hinting fault counts and placement decisions are both recorded in the same array which distorts the samples in an unpredictable fashion. The values linearly accumulate during the scan and then decay creating a sawtooth-like pattern in the per-node counts. It also means that placement decisions are time sensitive. At best it means that it is very difficult to state that the buffer holds a decaying average of past faulting behaviour. At worst, it can confuse the load balancer if it sees one node with an artifically high count due to very recent faulting activity and may create a bouncing effect. This patch adds a second array. numa_faults stores the historical data which is used for placement decisions. numa_faults_buffer holds the fault activity during the current scan window. When the scan completes, numa_faults decays and the values from numa_faults_buffer are copied across. Signed-off-by:
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Mel Gorman authored
This patch selects a preferred node for a task to run on based on the NUMA hinting faults. This information is later used to migrate tasks towards the node during balancing. Signed-off-by:
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Mel Gorman authored
This patch tracks what nodes numa hinting faults were incurred on. This information is later used to schedule a task on the node storing the pages most frequently faulted by the task. Signed-off-by:
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Mel Gorman authored
NUMA PTE scanning slows if a NUMA hinting fault was trapped and no page was migrated. For long-lived but idle processes there may be no faults but the scan rate will be high and just waste CPU. This patch will slow the scan rate for processes that are not trapping faults. Signed-off-by:
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Mel Gorman authored
The NUMA PTE scan rate is controlled with a combination of the numa_balancing_scan_period_min, numa_balancing_scan_period_max and numa_balancing_scan_size. This scan rate is independent of the size of the task and as an aside it is further complicated by the fact that numa_balancing_scan_size controls how many pages are marked pte_numa and not how much virtual memory is scanned. In combination, it is almost impossible to meaningfully tune the min and max scan periods and reasoning about performance is complex when the time to complete a full scan is is partially a function of the tasks memory size. This patch alters the semantic of the min and max tunables to be about tuning the length time it takes to complete a scan of a tasks occupied virtual address space. Conceptually this is a lot easier to understand. There is a "sanity" check to ensure the scan rate is never extremely fast based on the amount of virtual memory that should be scanned in a second. The default of 2.5G seems arbitrary but it is to have the maximum scan rate after the patch roughly match the maximum scan rate before the patch was applied. On a similar note, numa_scan_period is in milliseconds and not jiffies. Properly placed pages slow the scanning rate but adding 10 jiffies to numa_scan_period means that the rate scanning slows depends on HZ which is confusing. Get rid of the jiffies_to_msec conversion and treat it as ms. Signed-off-by:
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Mel Gorman authored
Scan delay logic and resets are currently initialised to start scanning immediately instead of delaying properly. Initialise them properly at fork time and catch when a new mm has been allocated. Signed-off-by:
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Mel Gorman authored
PTE scanning and NUMA hinting fault handling is expensive so commit 5bca2303 ("mm: sched: numa: Delay PTE scanning until a task is scheduled on a new node") deferred the PTE scan until a task had been scheduled on another node. The problem is that in the purely shared memory case that this may never happen and no NUMA hinting fault information will be captured. We are not ruling out the possibility that something better can be done here but for now, this patch needs to be reverted and depend entirely on the scan_delay to avoid punishing short-lived processes. Signed-off-by:
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Peter Zijlstra authored
Avoiding marking PTEs pte_numa because a particular NUMA node is migrate rate limited sees like a bad idea. Even if this node can't migrate anymore other nodes might and we want up-to-date information to do balance decisions. We already rate limit the actual migrations, this should leave enough bandwidth to allow the non-migrating scanning. I think its important we keep up-to-date information if we're going to do placement based on it. Signed-off-by:
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Peter Zijlstra authored
With a trace_printk("working\n"); right after the cmpxchg in task_numa_work() we can see that of a 4 thread process, its always the same task winning the race and doing the protection change. This is a problem since the task doing the protection change has a penalty for taking faults -- it is busy when marking the PTEs. If its always the same task the ->numa_faults[] get severely skewed. Avoid this by delaying the task doing the protection change such that it is unlikely to win the privilege again. Before: root@interlagos:~# grep "thread 0/.*working" /debug/tracing/trace | tail -15 thread 0/0-3232 [022] .... 212.787402: task_numa_work: working thread 0/0-3232 [022] .... 212.888473: task_numa_work: working thread 0/0-3232 [022] .... 212.989538: task_numa_work: working thread 0/0-3232 [022] .... 213.090602: task_numa_work: working thread 0/0-3232 [022] .... 213.191667: task_numa_work: working thread 0/0-3232 [022] .... 213.292734: task_numa_work: working thread 0/0-3232 [022] .... 213.393804: task_numa_work: working thread 0/0-3232 [022] .... 213.494869: task_numa_work: working thread 0/0-3232 [022] .... 213.596937: task_numa_work: working thread 0/0-3232 [022] .... 213.699000: task_numa_work: working thread 0/0-3232 [022] .... 213.801067: task_numa_work: working thread 0/0-3232 [022] .... 213.903155: task_numa_work: working thread 0/0-3232 [022] .... 214.005201: task_numa_work: working thread 0/0-3232 [022] .... 214.107266: task_numa_work: working thread 0/0-3232 [022] .... 214.209342: task_numa_work: working After: root@interlagos:~# grep "thread 0/.*working" /debug/tracing/trace | tail -15 thread 0/0-3253 [005] .... 136.865051: task_numa_work: working thread 0/2-3255 [026] .... 136.965134: task_numa_work: working thread 0/3-3256 [024] .... 137.065217: task_numa_work: working thread 0/3-3256 [024] .... 137.165302: task_numa_work: working thread 0/3-3256 [024] .... 137.265382: task_numa_work: working thread 0/0-3253 [004] .... 137.366465: task_numa_work: working thread 0/2-3255 [026] .... 137.466549: task_numa_work: working thread 0/0-3253 [004] .... 137.566629: task_numa_work: working thread 0/0-3253 [004] .... 137.666711: task_numa_work: working thread 0/1-3254 [028] .... 137.766799: task_numa_work: working thread 0/0-3253 [004] .... 137.866876: task_numa_work: working thread 0/2-3255 [026] .... 137.966960: task_numa_work: working thread 0/1-3254 [028] .... 138.067041: task_numa_work: working thread 0/2-3255 [026] .... 138.167123: task_numa_work: working thread 0/3-3256 [024] .... 138.267207: task_numa_work: working Signed-off-by: -
Mel Gorman authored
The zero page is not replicated between nodes and is often shared between processes. The data is read-only and likely to be cached in local CPUs if heavily accessed meaning that the remote memory access cost is less of a concern. This patch prevents trapping faults on the zero pages. For tasks using the zero page this will reduce the number of PTE updates, TLB flushes and hinting faults. Signed-off-by:
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Mel Gorman authored
NUMA PTE scanning is expensive both in terms of the scanning itself and the TLB flush if there are any updates. The TLB flush is avoided if no PTEs are updated but there is a bug where transhuge PMDs are considered to be updated even if they were already pmd_numa. This patch addresses the problem and TLB flushes should be reduced. Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> Reviewed-by:
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Mel Gorman authored
NUMA PTE scanning is expensive both in terms of the scanning itself and the TLB flush if there are any updates. Currently non-present PTEs are accounted for as an update and incurring a TLB flush where it is only necessary for anonymous migration entries. This patch addresses the problem and should reduce TLB flushes. Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> Reviewed-by:
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Mel Gorman authored
A THP PMD update is accounted for as 512 pages updated in vmstat. This is large difference when estimating the cost of automatic NUMA balancing and can be misleading when comparing results that had collapsed versus split THP. This patch addresses the accounting issue. Signed-off-by:
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Mel Gorman authored
THP migration uses the page lock to guard against parallel allocations but there are cases like this still open Task A Task B --------------------- --------------------- do_huge_pmd_numa_page do_huge_pmd_numa_page lock_page mpol_misplaced == -1 unlock_page goto clear_pmdnuma lock_page mpol_misplaced == 2 migrate_misplaced_transhuge pmd = pmd_mknonnuma set_pmd_at During hours of testing, one crashed with weird errors and while I have no direct evidence, I suspect something like the race above happened. This patch extends the page lock to being held until the pmd_numa is cleared to prevent migration starting in parallel while the pmd_numa is being cleared. It also flushes the old pmd entry and orders pagetable insertion before rmap insertion. Signed-off-by:
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Mel Gorman authored
There are three callers of task_numa_fault(): - do_huge_pmd_numa_page(): Accounts against the current node, not the node where the page resides, unless we migrated, in which case it accounts against the node we migrated to. - do_numa_page(): Accounts against the current node, not the node where the page resides, unless we migrated, in which case it accounts against the node we migrated to. - do_pmd_numa_page(): Accounts not at all when the page isn't migrated, otherwise accounts against the node we migrated towards. This seems wrong to me; all three sites should have the same sementaics, furthermore we should accounts against where the page really is, we already know where the task is. So modify all three sites to always account; we did after all receive the fault; and always account to where the page is after migration, regardless of success. They all still differ on when they clear the PTE/PMD; ideally that would get sorted too. Signed-off-by: -
Mel Gorman authored
THP migrations are serialised by the page lock but on its own that does not prevent THP splits. If the page is split during THP migration then the pmd_same checks will prevent page table corruption but the unlock page and other fix-ups potentially will cause corruption. This patch takes the anon_vma lock to prevent parallel splits during migration. Signed-off-by:
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Mel Gorman authored
The locking for migrating THP is unusual. While normal page migration prevents parallel accesses using a migration PTE, THP migration relies on a combination of the page_table_lock, the page lock and the existance of the NUMA hinting PTE to guarantee safety but there is a bug in the scheme. If a THP page is currently being migrated and another thread traps a fault on the same page it checks if the page is misplaced. If it is not, then pmd_numa is cleared. The problem is that it checks if the page is misplaced without holding the page lock meaning that the racing thread can be migrating the THP when the second thread clears the NUMA bit and faults a stale page. This patch checks if the page is potentially being migrated and stalls using the lock_page if it is potentially being migrated before checking if the page is misplaced or not. Signed-off-by:
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Mel Gorman authored
If another task handled a hinting fault in parallel then do not double account for it. Signed-off-by:
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Peter Zijlstra authored
Fix a 80 column violation and a PTE vs PMD reference. Signed-off-by:
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Mel Gorman authored
Signed-off-by:
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Ingo Molnar authored
Merge Linux v3.12-rc4 to fix a conflict and also to refresh the tree before applying more scheduler patches. Conflicts: arch/avr32/include/asm/Kbuild Signed-off-by:
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- 06 Oct, 2013 1 commit
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Linus Torvalds authored
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