linux-kernel - Re: [PATCH v4 10/16] locking/rwsem: Wake up almost all readers in wait queue

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Message-ID: <20190416165038.xfjserwdhli6p222@linux-r8p5>
Date:   Tue, 16 Apr 2019 09:50:38 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 10/16] locking/rwsem: Wake up almost all readers in
 wait queue

On Sat, 13 Apr 2019, Waiman Long wrote:
>+/*
>+ * We limit the maximum number of readers that can be woken up for a
>+ * wake-up call to not penalizing the waking thread for spending too
>+ * much time doing it.
>+ */
>+#define MAX_READERS_WAKEUP	0x100

Although with wake_q this is not really so... Could it at least be
rewritten, dunno something like so:

/*
 * Magic number to batch-wakeup waiting readers, even when writers
 * are also present in the queue. This both limits the amount of
 * work the waking thread must do (albeit wake_q)  and also prevents
 * any potential counter overflow, however unlikely.
 */

I'm still not crazy about this artificial limit for the readers-only
case, but won't argue. I certainly like the reader/writer case.

Thanks,
Davidlohr