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

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Date:   Tue, 16 Apr 2019 13:37:38 -0400
From:   Waiman Long <longman@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>
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 04/16/2019 12:50 PM, Davidlohr Bueso wrote:
> 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.
> */
>

The wording looks good to me. Will modify that for the next version.
BTW, wake_q_add() has low overhead and so the lock hold time should be
short. Outside the wait_lock, wake_up_q() still has a high overhead if
there are many tasks to be woken up.

Cheers,
Longman