linux-kernel - Re: [PATCH] blk-mq: fix incorrect rq start_time_ns and alloc_time

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Message-ID: <ZIJco7_jaVaxpxSR@slm.duckdns.org>
Date:   Thu, 8 Jun 2023 12:56:35 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Chengming Zhou <chengming.zhou@...ux.dev>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [PATCH] blk-mq: fix incorrect rq start_time_ns and alloc_time_ns
 after throttled

Hello,

On Tue, Jun 06, 2023 at 06:22:28PM +0800, Chengming Zhou wrote:
...
> But for plug batched allocation introduced by the commit 47c122e35d7e
> ("block: pre-allocate requests if plug is started and is a batch"), we can
> rq_qos_throttle() after the allocation of the request. This is what the
> blk_mq_get_cached_request() does.
> 
> In this case, the cached request alloc_time_ns or start_time_ns is much ahead
> if block in any qos ->throttle().

Ah, okay, that's problematic.

> >> This patch add nr_flush counter in blk_plug, so we can tell if the task
> >> has throttled in any qos ->throttle(), in which case we need to correct
> >> the rq start_time_ns and alloc_time_ns.
> >>
> >> Another solution may be make rq_qos_throttle() return bool to indicate
> >> if it has throttled in any qos ->throttle(). But this need more changes.
> >>
> >> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> > 
> > Depending on the flush behavior and adjusting alloc_time_ns seems fragile to
> > me and will likely confuse other users of alloc_time_ns too.
> 
> I agree with you, this code is not good. My basic idea is to adjust the cached
> request alloc_time_ns and start_time_ns when throttled.

Would it make sense to skip setting the alloc_time_ns during pre-allocation
and set it later when the pre-allocated rq is actually used? That should
jive better.

Thanks.

-- 
tejun