Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Pavel Begunkov <asml.silence@gmail.com>,
        Ingo Molnar <mingo@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <a4996ae7-ac0a-447b-49b2-7e96275aad29@kernel.dk>
 <20190923083549.GA42487@gmail.com>
 <c15b2d54-c722-8fb4-266f-b589c1a21aa5@gmail.com>
 <df612e90-8999-0085-d2d6-4418e044e429@gmail.com>
 <731b2087-7786-5374-68ff-8cba42f0cd68@kernel.dk>
 <759b9b48-1de3-1d43-3e39-9c530bfffaa0@kernel.dk>
 <43244626-9cfd-0c0b-e7a1-878363712ef3@gmail.com>
 <f2608e3d-bb4e-9984-79e8-a2ab4f855c7f@kernel.dk>
 <b999490f-6138-b685-5472-5cd1843b747d@kernel.dk>
 <ed37058b-ee96-7d44-1dc7-d2c48e2ac23f@kernel.dk>
 <20190924094942.GN2349@hirez.programming.kicks-ass.net>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <6f935fb9-6ebd-1df1-0cd0-69e34a16fa7e@kernel.dk>
Date:   Tue, 24 Sep 2019 12:13:15 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <20190924094942.GN2349@hirez.programming.kicks-ass.net>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 9/24/19 3:49 AM, Peter Zijlstra wrote:
> On Tue, Sep 24, 2019 at 10:36:28AM +0200, Jens Axboe wrote:
> 
>> +struct io_wait_queue {
>> +	struct wait_queue_entry wq;
>> +	struct io_ring_ctx *ctx;
>> +	struct task_struct *task;
> 
> wq.private is where the normal waitqueue stores the task pointer.
> 
> (I'm going to rename that)

If you do that, then we can just base the io_uring parts on that. 

>> +	unsigned to_wait;
>> +	unsigned nr_timeouts;
>> +};
>> +
>> +static inline bool io_should_wake(struct io_wait_queue *iowq)
>> +{
>> +	struct io_ring_ctx *ctx = iowq->ctx;
>> +
>> +	/*
>> +	 * Wake up if we have enough events, or if a timeout occured since we
>> +	 * started waiting. For timeouts, we always want to return to userspace,
>> +	 * regardless of event count.
>> +	 */
>> +	return io_cqring_events(ctx->rings) >= iowq->to_wait ||
>> +			atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
>> +}
>> +
>> +static int io_wake_function(struct wait_queue_entry *curr, unsigned int mode,
>> +			    int wake_flags, void *key)
>> +{
>> +	struct io_wait_queue *iowq = container_of(curr, struct io_wait_queue,
>> +							wq);
>> +
>> +	if (io_should_wake(iowq)) {
>> +		list_del_init(&curr->entry);
>> +		wake_up_process(iowq->task);
> 
> Then you can use autoremove_wake_function() here.
> 
>> +		return 1;
>> +	}
>> +
>> +	return -1;
>> +}
> 
> Ideally we'd get wait_event()'s @cond in a custom wake function. Then we
> can _always_ do this.
> 
> This is one I'd love to have lambda functions for. It would actually
> work with GCC nested functions, because the wake function will always be
> in scope, but we can't use those in the kernel for other reasons :/

I'll be happy enough if I can just call autoremove_wake_function(), I
think that will simplify the case enough for io_uring to not really make
me care too much about going further. I'll leave that to you, if you
have the desire :-)

-- 
Jens Axboe