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    fs/epoll: drop ovflist branch prediction · 9cb8f808
    Davidlohr Bueso authored
    [ Upstream commit 76699a67 ]
    
    The ep->ovflist is a secondary ready-list to temporarily store events
    that might occur when doing sproc without holding the ep->wq.lock.  This
    accounts for every time we check for ready events and also send events
    back to userspace; both callbacks, particularly the latter because of
    copy_to_user, can account for a non-trivial time.
    
    As such, the unlikely() check to see if the pointer is being used, seems
    both misleading and sub-optimal.  In fact, we go to an awful lot of
    trouble to sync both lists, and populating the ovflist is far from an
    uncommon scenario.
    
    For example, profiling a concurrent epoll_wait(2) benchmark, with
    CONFIG_PROFILE_ANNOTATED_BRANCHES shows that for a two threads a 33%
    incorrect rate was seen; and when incrementally increasing the number of
    epoll instances (which is used, for example for multiple queuing load
    balancing models), up to a 90% incorrect rate was seen.
    
    Similarly, by deleting the prediction, 3% throughput boost was seen
    across incremental threads.
    
    Link: http://lkml.kernel.org/r/20181108051006.18751-4-dave@stgolabs.net
    
    
    Signed-off-by: default avatarDavidlohr Bueso <dbueso@suse.de>
    Reviewed-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Jason Baron <jbaron@akamai.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
    9cb8f808