linux-kernel - Re: [PATCH] eventfd: support delayed wakeup for non-semaphore eventfd to reduce cpu utilization

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Message-ID: <817984a2-570c-cb23-4121-0d75005ebd4d@kernel.dk>
Date:   Mon, 17 Apr 2023 08:38:15 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     wenyang.linux@...mail.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Dylan Yudaken <dylany@...com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Paolo Bonzini <pbonzini@...hat.com>, Fu Wei <wefu@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] eventfd: support delayed wakeup for non-semaphore eventfd
 to reduce cpu utilization

On 4/16/23 5:31?AM, wenyang.linux@...mail.com wrote:
> From: Wen Yang <wenyang.linux@...mail.com>
> 
> For the NON SEMAPHORE eventfd, if it's counter has a nonzero value,
> then a read(2) returns 8 bytes containing that value, and the counter's
> value is reset to zero. Therefore, in the NON SEMAPHORE scenario,
> N event_writes vs ONE event_read is possible.
> 
> However, the current implementation wakes up the read thread immediately
> in eventfd_write so that the cpu utilization increases unnecessarily.
> 
> By adding a configurable delay after eventfd_write, these unnecessary
> wakeup operations are avoided, thereby reducing cpu utilization.

What's the real world use case of this, and what would the expected
delay be there? With using a delayed work item for this, there's
certainly a pretty wide grey zone in terms of delay where this would
perform considerably worse than not doing any delayed wakeups at all.

-- 
Jens Axboe