mm,oom: speed up select_bad_process() loop

mm,oom: speed up select_bad_process() loop

Since commit 3a5dda7a17cf ("oom: prevent unnecessary oom kills or kernel
panics"), select_bad_process() is using for_each_process_thread().

Since oom_unkillable_task() scans all threads in the caller's thread
group and oom_task_origin() scans signal_struct of the caller's thread
group, we don't need to call oom_unkillable_task() and oom_task_origin()
on each thread.  Also, since !mm test will be done later at
oom_badness(), we don't need to do !mm test on each thread.  Therefore,
we only need to do TIF_MEMDIE test on each thread.

Although the original code was correct it was quite inefficient because
each thread group was scanned num_threads times which can be a lot
especially with processes with many threads.  Even though the OOM is
extremely cold path it is always good to be as effective as possible
when we are inside rcu_read_lock() - aka unpreemptible context.

If we track number of TIF_MEMDIE threads inside signal_struct, we don't
need to do TIF_MEMDIE test on each thread.  This will allow
select_bad_process() to use for_each_process().

This patch adds a counter to signal_struct for tracking how many
TIF_MEMDIE threads are in a given thread group, and check it at
oom_scan_process_thread() so that select_bad_process() can use
for_each_process() rather than for_each_process_thread().

[mhocko@suse.com: do not blow the signal_struct size]
Link: http://lkml.kernel.org/r/20160520075035.GF19172@dhcp22.suse.cz
Link: http://lkml.kernel.org/r/201605182230.IDC73435.MVSOHLFOQFOJtF@I-love.SAKURA.ne.jp
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>