linux-kernel - Re: [PATCH v1] fs/epoll: use a per-cpu counter for user's watches count

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Date:   Wed, 4 Aug 2021 08:22:22 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Anton Blanchard <anton@...abs.org>
Subject: Re: [PATCH v1] fs/epoll: use a per-cpu counter for user's watches
 count

On Mon, Aug 02, 2021 at 01:20:13PM +1000, Nicholas Piggin wrote:
> This counter tracks the number of watches a user has, to compare against
> the 'max_user_watches' limit. This causes a scalability bottleneck on
> SPECjbb2015 on large systems as there is only one user. Changing to a
> per-cpu counter increases throughput of the benchmark by about 30% on a
> 16-socket, > 1000 thread system.
> 
> Reported-by: Anton Blanchard <anton@...abs.org>
> Signed-off-by: Nicholas Piggin <npiggin@...il.com>

With all tinyconfig builds (and all other builds with CONFIG_EPOLL=n),
this patch results in:

kernel/user.c: In function 'free_user':
kernel/user.c:141:35: error: 'struct user_struct' has no member named 'epoll_watches'
  141 |         percpu_counter_destroy(&up->epoll_watches);
      |                                   ^~
kernel/user.c: In function 'alloc_uid':
kernel/user.c:189:45: error: 'struct user_struct' has no member named 'epoll_watches'
  189 |                 if (percpu_counter_init(&new->epoll_watches, 0, GFP_KERNEL)) {
      |                                             ^~
kernel/user.c:203:52: error: 'struct user_struct' has no member named 'epoll_watches'
  203 |                         percpu_counter_destroy(&new->epoll_watches);
      |                                                    ^~
kernel/user.c: In function 'uid_cache_init':
kernel/user.c:225:43: error: 'struct user_struct' has no member named 'epoll_watches'
  225 |         if (percpu_counter_init(&root_user.epoll_watches, 0, GFP_KERNEL))
      |                                           ^

Guenter