• Suren Baghdasaryan's avatar
    mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary · 67197a4f
    Suren Baghdasaryan authored
    Currently __set_oom_adj loops through all processes in the system to keep
    oom_score_adj and oom_score_adj_min in sync between processes sharing
    their mm.  This is done for any task with more that one mm_users, which
    includes processes with multiple threads (sharing mm and signals).
    However for such processes the loop is unnecessary because their signal
    structure is shared as well.
    
    Android updates oom_score_adj whenever a tasks changes its role
    (background/foreground/...) or binds to/unbinds from a service, making it
    more/less important.  Such operation can happen frequently.  We noticed
    that updates to oom_score_adj became more expensive and after further
    investigation found out that the patch mentioned in "Fixes" introduced a
    regression.  Using Pixel 4 with a typical Android workload, write time to
    oom_score_adj increased from ~3.57us to ~362us.  Moreover this regression
    linearly depends on the number of multi-threaded processes running on the
    system.
    
    Mark the mm with a new MMF_MULTIPROCESS flag bit when task is created with
    (CLONE_VM && !CLONE_THREAD && !CLONE_VFORK).  Change __set_oom_adj to use
    MMF_MULTIPROCESS instead of mm_users to decide whether oom_score_adj
    update should be synchronized between multiple processes.  To prevent
    races between clone() and __set_oom_adj(), when oom_score_adj of the
    process being cloned might be modified from userspace, we use
    oom_adj_mutex.  Its scope is changed to global.
    
    The combination of (CLONE_VM && !CLONE_THREAD) is rarely used except for
    the case of vfork().  To prevent performance regressions of vfork(), we
    skip taking oom_adj_mutex and setting MMF_MULTIPROCESS when CLONE_VFORK is
    specified.  Clearing the MMF_MULTIPROCESS flag (when the last process
    sharing the mm exits) is left out of this patch to keep it simple and
    because it is believed that this threading model is rare.  Should there
    ever be a need for optimizing that case as well, it can be done by hooking
    into the exit path, likely following the mm_update_next_owner pattern.
    
    With the combination of (CLONE_VM && !CLONE_THREAD && !CLONE_VFORK) being
    quite rare, the regression is gone after the change is applied.
    
    [surenb@google.com: v3]
      Link: https://lkml.kernel.org/r/20200902012558.2335613-1-surenb@google.com
    
    Fixes: 44a70ade ("mm, oom_adj: make sure processes sharing mm have same view of oom_score_adj")
    Reported-by: default avatarTim Murray <timmurray@google.com>
    Suggested-by: default avatarMichal Hocko <mhocko@kernel.org>
    Signed-off-by: default avatarSuren Baghdasaryan <surenb@google.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Acked-by: default avatarChristian Brauner <christian.brauner@ubuntu.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Eugene Syromiatnikov <esyr@redhat.com>
    Cc: Christian Kellner <christian@kellner.me>
    Cc: Adrian Reber <areber@redhat.com>
    Cc: Shakeel Butt <shakeelb@google.com>
    Cc: Aleksa Sarai <cyphar@cyphar.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: "Eric W. Biederman" <ebiederm@xmission.com>
    Cc: Alexey Gladkov <gladkov.alexey@gmail.com>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
    Cc: Andrei Vagin <avagin@gmail.com>
    Cc: Bernd Edlinger <bernd.edlinger@hotmail.de>
    Cc: John Johansen <john.johansen@canonical.com>
    Cc: Yafang Shao <laoar.shao@gmail.com>
    Link: https://lkml.kernel.org/r/20200824153036.3201505-1-surenb@google.comDebugged-by: default avatarMinchan Kim <minchan@kernel.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    67197a4f
fork.c 74 KB