linux-kernel - Re: [PATCH v3] block: Limit number of items taken from the I/O scheduler in one go

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Message-ID: <20200424153940.GA17253@lst.de>
Date:   Fri, 24 Apr 2020 17:39:40 +0200
From:   Christoph Hellwig <hch@....de>
To:     Salman Qazi <sqazi@...gle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...hat.com>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jesse Barnes <jsbarnes@...gle.com>,
        Gwendal Grignou <gwendal@...gle.com>,
        Hannes Reinecke <hare@...e.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3] block: Limit number of items taken from the I/O
 scheduler in one go

> +	/*
> +	 * A return of -EAGAIN is an indication that hctx->dispatch is not
> +	 * empty and we must run again in order to avoid starving flushes.
> +	 */
> +	if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN) {
> +		if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN)
> +			blk_mq_run_hw_queue(hctx, true);
> +	}

The comment doesn't explain why we give up after the second attempt and
do a blk_mq_run_hw_queue instead.