Re: [PATCH] proc: Avoid a thundering herd of threads freeing proc dentries

[Masahiro Yamada]

On Fri, Jun 19, 2020 at 11:14 PM Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
>
>
> Junxiao Bi <junxiao.bi@xxxxxxxxxx> reported:
> > When debugging some performance issue, i found that thousands of threads exit
> > around same time could cause a severe spin lock contention on proc dentry
> > "/proc/$parent_process_pid/task/", that's because threads needs to clean up
> > their pid file from that dir when exit.
>
> Matthew Wilcox <willy@xxxxxxxxxxxxx> reported:
> > We've looked at a few different ways of fixing this problem.
>
> The flushing of the proc dentries from the dcache is an optmization,
> and is not necessary for correctness.  Eventually cache pressure will
> cause the dentries to be freed even if no flushing happens.  Some
> light testing when I refactored the proc flushg[1] indicated that at
> least the memory footprint is easily measurable.
>
> An optimization that causes a performance problem due to a thundering
> herd of threads is no real optimization.
>
> Modify the code to only flush the /proc/<tgid>/ directory when all
> threads in a process are killed at once.  This continues to flush
> practically everything when the process is reaped as the threads live
> under /proc/<tgid>/task/<tid>.
>
> There is a rare possibility that a debugger will access /proc/<tid>/,
> which this change will no longer flush, but I believe such accesses
> are sufficiently rare to not be observed in practice.
>
> [1] 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc")
> Link: https://lkml.kernel.org/r/54091fc0-ca46-2186-97a8-d1f3c4f3877b@xxxxxxxxxx


> Reported-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>

I did not report this.





> Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---


-- 
Best Regards
Masahiro Yamada