From: Bart Van Assche <bvanassche@acm.org>
To: Salman Qazi <sqazi@google.com>, Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: Jesse Barnes <jsbarnes@google.com>,
Gwendal Grignou <gwendal@google.com>,
Christoph Hellwig <hch@lst.de>, Ming Lei <ming.lei@redhat.com>,
Hannes Reinecke <hare@suse.com>
Subject: Re: [PATCH] block: Limit number of items taken from the I/O scheduler in one go
Date: Mon, 3 Feb 2020 19:47:02 -0800 [thread overview]
Message-ID: <6499bf21-df25-3881-4252-8c51a9785bb7@acm.org> (raw)
In-Reply-To: <20200203205950.127629-1-sqazi@google.com>
On 2020-02-03 12:59, Salman Qazi wrote:
> We observed that it is possible for a flush to bypass the I/O
> scheduler and get added to hctx->dispatch in blk_mq_sched_bypass_insert.
> This can happen while a kworker is running blk_mq_do_dispatch_sched call
> in blk_mq_sched_dispatch_requests.
>
> However, the blk_mq_do_dispatch_sched call doesn't end in bounded time.
> As a result, the flush can sit there indefinitely, as the I/O scheduler
> feeds an arbitrary number of requests to the hardware.
>
> The solution is to periodically poll hctx->dispatch in
> blk_mq_do_dispatch_sched, to put a bound on the latency of the commands
> sitting there.
(added Christoph, Ming and Hannes to the Cc-list)
Thank you for having posted a patch; that really helps.
I see that my name occurs first in the "To:" list. Since Jens is the
block layer maintainer I think Jens should have been mentioned first.
In version v4.20 of the Linux kernel I found the following in the legacy
block layer code:
* From blk_insert_flush():
list_add_tail(&rq->queuelist, &q->queue_head);
* From elv_next_request():
list_for_each_entry(rq, &q->queue_head, queuelist)
I think this means that the legacy block layer sent flush requests to
the scheduler instead of directly to the block driver. How about
modifying the blk-mq code such that it mimics that approach? I'm asking
this because this patch, although the code looks clean, doesn't seem the
best solution to me.
> + if (count > 0 && count % q->max_sched_batch == 0 &&
> + !list_empty_careful(&hctx->dispatch))
> + break;
A modulo operation in the hot path? Please don't do that.
> +static ssize_t queue_max_sched_batch_store(struct request_queue *q,
> + const char *page,
> + size_t count)
> +{
> + int err, val;
> +
> + if (!q->mq_ops)
> + return -EINVAL;
> +
> + err = kstrtoint(page, 10, &val);
> + if (err < 0)
> + return err;
> +
> + if (val <= 0)
> + return -EINVAL;
> +
> + q->max_sched_batch = val;
> +
> + return count;
> +}
Has it been considered to use kstrtouint() instead of checking whether
the value returned by kstrtoint() is positive?
> + int max_sched_batch;
unsigned int?
Thanks,
Bart.
next prev parent reply other threads:[~2020-02-04 3:47 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-30 19:34 Hung tasks with multiple partitions Salman Qazi
2020-01-30 20:49 ` Bart Van Assche
2020-01-30 21:02 ` Salman Qazi
[not found] ` <20200203204554.119849-1-sqazi@google.com>
2020-02-03 20:59 ` [PATCH] block: Limit number of items taken from the I/O scheduler in one go Salman Qazi
2020-02-04 3:47 ` Bart Van Assche [this message]
2020-02-04 9:20 ` Ming Lei
2020-02-04 18:26 ` Salman Qazi
2020-02-04 19:37 ` Salman Qazi
2020-02-05 4:55 ` Ming Lei
2020-02-05 19:57 ` Salman Qazi
2020-02-06 10:18 ` Ming Lei
2020-02-06 21:12 ` Salman Qazi
2020-02-07 2:07 ` Ming Lei
2020-02-07 15:26 ` Bart Van Assche
2020-02-07 18:45 ` Salman Qazi
2020-02-07 19:04 ` Salman Qazi
2020-02-07 20:19 ` Bart Van Assche
2020-02-07 20:37 ` Salman Qazi
2020-04-20 16:42 ` Doug Anderson
2020-04-23 20:13 ` Jesse Barnes
2020-04-23 20:34 ` Jens Axboe
2020-04-23 20:40 ` Salman Qazi
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