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Date:   Mon, 17 Apr 2023 08:38:15 -0600
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Subject: Re: [PATCH] eventfd: support delayed wakeup for non-semaphore eventfd
 to reduce cpu utilization
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To:     wenyang.linux@foxmail.com,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>
Cc:     Christoph Hellwig <hch@lst.de>, Dylan Yudaken <dylany@fb.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Paolo Bonzini <pbonzini@redhat.com>, Fu Wei <wefu@redhat.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <tencent_AF886EF226FD9F39D28FE4D9A94A95FA2605@qq.com>
From:   Jens Axboe <axboe@kernel.dk>
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On 4/16/23 5:31?AM, wenyang.linux@foxmail.com wrote:
> From: Wen Yang <wenyang.linux@foxmail.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