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    • Tvrtko Ursulin's avatar
      drm/i915: Move execlists irq handler to a bottom half · 27af5eea
      Tvrtko Ursulin authored
      Doing a lot of work in the interrupt handler introduces huge
      latencies to the system as a whole.
      
      Most dramatic effect can be seen by running an all engine
      stress test like igt/gem_exec_nop/all where, when the kernel
      config is lean enough, the whole system can be brought into
      multi-second periods of complete non-interactivty. That can
      look for example like this:
      
       NMI watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [kworker/u8:3:143]
       Modules linked in: [redacted for brevity]
       CPU: 0 PID: 143 Comm: kworker/u8:3 Tainted: G     U       L  4.5.0-160321+ #183
       Hardware name: Intel Corporation Broadwell Client platform/WhiteTip Mountain 1
       Workqueue: i915 gen6_pm_rps_work [i915]
       task: ffff8800aae88000 ti: ffff8800aae90000 task.ti: ffff8800aae90000
       RIP: 0010:[<ffffffff8104a3c2>]  [<ffffffff8104a3c2>] __do_softirq+0x72/0x1d0
       RSP: 0000:ffff88014f403f38  EFLAGS: 00000206
       RAX: ffff8800aae94000 RBX: 0000000000000000 RCX: 00000000000006e0
       RDX: 0000000000000020 RSI: 0000000004208060 RDI: 0000000000215d80
       RBP: ffff88014f403f80 R08: 0000000b1b42c180 R09: 0000000000000022
       R10: 0000000000000004 R11: 00000000ffffffff R12: 000000000000a030
       R13: 0000000000000082 R14: ffff8800aa4d0080 R15: 0000000000000082
       FS:  0000000000000000(0000) GS:ffff88014f400000(0000) knlGS:0000000000000000
       CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
       CR2: 00007fa53b90c000 CR3: 0000000001a0a000 CR4: 00000000001406f0
       DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
       DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
       Stack:
        042080601b33869f ffff8800aae94000 00000000fffc2678 ffff88010000000a
        0000000000000000 000000000000a030 0000000000005302 ffff8800aa4d0080
        0000000000000206 ffff88014f403f90 ffffffff8104a716 ffff88014f403fa8
       Call Trace:
        <IRQ>
        [<ffffffff8104a716>] irq_exit+0x86/0x90
        [<ffffffff81031e7d>] smp_apic_timer_interrupt+0x3d/0x50
        [<ffffffff814f3eac>] apic_timer_interrupt+0x7c/0x90
        <EOI>
        [<ffffffffa01c5b40>] ? gen8_write64+0x1a0/0x1a0 [i915]
        [<ffffffff814f2b39>] ? _raw_spin_unlock_irqrestore+0x9/0x20
        [<ffffffffa01c5c44>] gen8_write32+0x104/0x1a0 [i915]
        [<ffffffff8132c6a2>] ? n_tty_receive_buf_common+0x372/0xae0
        [<ffffffffa017cc9e>] gen6_set_rps_thresholds+0x1be/0x330 [i915]
        [<ffffffffa017eaf0>] gen6_set_rps+0x70/0x200 [i915]
        [<ffffffffa0185375>] intel_set_rps+0x25/0x30 [i915]
        [<ffffffffa01768fd>] gen6_pm_rps_work+0x10d/0x2e0 [i915]
        [<ffffffff81063852>] ? finish_task_switch+0x72/0x1c0
        [<ffffffff8105ab29>] process_one_work+0x139/0x350
        [<ffffffff8105b186>] worker_thread+0x126/0x490
        [<ffffffff8105b060>] ? rescuer_thread+0x320/0x320
        [<ffffffff8105fa64>] kthread+0xc4/0xe0
        [<ffffffff8105f9a0>] ? kthread_create_on_node+0x170/0x170
        [<ffffffff814f351f>] ret_from_fork+0x3f/0x70
        [<ffffffff8105f9a0>] ? kthread_create_on_node+0x170/0x170
      
      I could not explain, or find a code path, which would explain
      a +20 second lockup, but from some instrumentation it was
      apparent the interrupts off proportion of time was between
      10-25% under heavy load which is quite bad.
      
      When a interrupt "cliff" is reached, which was >~320k irq/s on
      my machine, the whole system goes into a terrible state of the
      above described multi-second lockups.
      
      By moving the GT interrupt handling to a tasklet in a most
      simple way, the problem above disappears completely.
      
      Testing the effect on sytem-wide latencies using
      igt/gem_syslatency shows the following before this patch:
      
      gem_syslatency: cycles=1532739, latency mean=416531.829us max=2499237us
      gem_syslatency: cycles=1839434, latency mean=1458099.157us max=4998944us
      gem_syslatency: cycles=1432570, latency mean=2688.451us max=1201185us
      gem_syslatency: cycles=1533543, latency mean=416520.499us max=2498886us
      
      This shows that the unrelated process is experiencing huge
      delays in its wake-up latency. After the patch the results
      look like this:
      
      gem_syslatency: cycles=808907, latency mean=53.133us max=1640us
      gem_syslatency: cycles=862154, latency mean=62.778us max=2117us
      gem_syslatency: cycles=856039, latency mean=58.079us max=2123us
      gem_syslatency: cycles=841683, latency mean=56.914us max=1667us
      
      Showing a huge improvement in the unrelated process wake-up
      latency. It also shows an approximate halving in the number
      of total empty batches submitted during the test. This may
      not be worrying since the test puts the driver under
      a very unrealistic load with ncpu threads doing empty batch
      submission to all GPU engines each.
      
      Another benefit compared to the hard-irq handling is that now
      work on all engines can be dispatched in parallel since we can
      have up to number of CPUs active tasklets. (While previously
      a single hard-irq would serially dispatch on one engine after
      another.)
      
      More interesting scenario with regards to throughput is
      "gem_latency -n 100" which  shows 25% better throughput and
      CPU usage, and 14% better dispatch latencies.
      
      I did not find any gains or regressions with Synmark2 or
      GLbench under light testing. More benchmarking is certainly
      required.
      
      v2:
         * execlists_lock should be taken as spin_lock_bh when
           queuing work from userspace now. (Chris Wilson)
         * uncore.lock must be taken with spin_lock_irq when
           submitting requests since that now runs from either
           softirq or process context.
      
      v3:
         * Expanded commit message with more testing data;
         * converted missed locking sites to _bh;
         * added execlist_lock comment. (Chris Wilson)
      
      v4:
         * Mention dispatch parallelism in commit. (Chris Wilson)
         * Do not hold uncore.lock over MMIO reads since the block
           is already serialised per-engine via the tasklet itself.
           (Chris Wilson)
         * intel_lrc_irq_handler should be static. (Chris Wilson)
         * Cancel/sync the tasklet on GPU reset. (Chris Wilson)
         * Document and WARN that tasklet cannot be active/pending
           on engine cleanup. (Chris Wilson/Imre Deak)
      Signed-off-by: default avatarTvrtko Ursulin <tvrtko.ursulin@intel.com>
      Cc: Chris Wilson <chris@chris-wilson.co.uk>
      Cc: Imre Deak <imre.deak@intel.com>
      Testcase: igt/gem_exec_nop/all
      Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94350Reviewed-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
      Link: http://patchwork.freedesktop.org/patch/msgid/1459768316-6670-1-git-send-email-tvrtko.ursulin@linux.intel.com
      27af5eea