linux-kernel - Re: [PATCH v5] do_wait: make PIDTYPE

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Message-Id: <20210312102207.a347e38db375226a78cc37bf@linux-foundation.org>
Date:   Fri, 12 Mar 2021 10:22:07 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jim Newsome <jnewsome@...project.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Christian Brauner <christian@...uner.io>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] do_wait: make PIDTYPE_PID case O(1) instead of O(n)

On Fri, 12 Mar 2021 11:38:55 -0600 Jim Newsome <jnewsome@...project.org> wrote:

> do_wait is an internal function used to implement waitpid, waitid,
> wait4, etc. To handle the general case, it does an O(n) linear scan of
> the thread group's children and tracees.
> 
> This patch adds a special-case when waiting on a pid to skip these scans
> and instead do an O(1) lookup. This improves performance when waiting on
> a pid from a thread group with many children and/or tracees.

Could we please see some performance testing results to permit us to
evaluate the value of this change?

Thanks.