From: Mike Galbraith <efault@gmx.de>
To: Peter Zijlstra <peterz@infradead.org>,
Bart Van Assche <Bart.VanAssche@wdc.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
"kernel-team@fb.com" <kernel-team@fb.com>,
"oleg@redhat.com" <oleg@redhat.com>, "hch@lst.de" <hch@lst.de>,
"axboe@kernel.dk" <axboe@kernel.dk>,
"jianchao.w.wang@oracle.com" <jianchao.w.wang@oracle.com>,
"osandov@fb.com" <osandov@fb.com>,
"tj@kernel.org" <tj@kernel.org>
Subject: Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme
Date: Fri, 15 Dec 2017 03:39:30 +0100 [thread overview]
Message-ID: <1513305570.893.7.camel@gmx.de> (raw)
In-Reply-To: <20171214215404.GK3326@worktop>
On Thu, 2017-12-14 at 22:54 +0100, Peter Zijlstra wrote:
> On Thu, Dec 14, 2017 at 09:42:48PM +0000, Bart Van Assche wrote:
>
> > Some time ago the block layer was changed to handle timeouts in thread context
> > instead of interrupt context. See also commit 287922eb0b18 ("block: defer
> > timeouts to a workqueue").
>
> That only makes it a little better:
>
> Task-A Worker
>
> write_seqcount_begin()
> blk_mq_rw_update_state(rq, IN_FLIGHT)
> blk_add_timer(rq)
> <timer>
> schedule_work()
> </timer>
> <context-switch to worker>
> read_seqcount_begin()
> while(seq & 1)
> cpu_relax();
>
>
> Now normally this isn't fatal because Worker will simply spin its entire
> time slice away and we'll eventually schedule our Task-A back in, which
> will complete the seqcount and things will work.
>
> But if, for some reason, our Worker was to have RT priority higher than
> our Task-A we'd be up some creek without no paddles.
Most kthreads, including kworkers, are very frequently SCHED_FIFO here.
-Mike
next prev parent reply other threads:[~2017-12-15 2:40 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-12 19:01 [PATCHSET v2] blk-mq: reimplement timeout handling Tejun Heo
2017-12-12 19:01 ` [PATCH 1/6] blk-mq: protect completion path with RCU Tejun Heo
2017-12-13 3:30 ` jianchao.wang
2017-12-13 16:13 ` Tejun Heo
2017-12-14 2:09 ` jianchao.wang
2017-12-14 17:01 ` Bart Van Assche
2017-12-14 18:14 ` tj
2017-12-12 19:01 ` [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme Tejun Heo
2017-12-12 21:37 ` Bart Van Assche
2017-12-12 21:44 ` tj
2017-12-13 5:07 ` jianchao.wang
2017-12-13 16:13 ` Tejun Heo
2017-12-14 18:51 ` Bart Van Assche
2017-12-14 19:19 ` tj
2017-12-14 21:13 ` Bart Van Assche
2017-12-15 13:30 ` tj
2017-12-14 20:20 ` Peter Zijlstra
2017-12-14 21:42 ` Bart Van Assche
2017-12-14 21:54 ` Peter Zijlstra
2017-12-15 2:12 ` jianchao.wang
2017-12-15 7:31 ` Peter Zijlstra
2017-12-15 15:14 ` jianchao.wang
2017-12-15 2:39 ` Mike Galbraith [this message]
2017-12-15 13:50 ` tj
2017-12-12 19:01 ` [PATCH 3/6] blk-mq: use blk_mq_rq_state() instead of testing REQ_ATOM_COMPLETE Tejun Heo
2017-12-12 19:01 ` [PATCH 4/6] blk-mq: make blk_abort_request() trigger timeout path Tejun Heo
2017-12-14 18:56 ` Bart Van Assche
2017-12-14 19:26 ` tj
2017-12-12 19:01 ` [PATCH 5/6] blk-mq: remove REQ_ATOM_COMPLETE usages from blk-mq Tejun Heo
2017-12-12 19:01 ` [PATCH 6/6] blk-mq: remove REQ_ATOM_STARTED Tejun Heo
2017-12-12 22:20 ` Bart Van Assche
2017-12-12 22:22 ` tj
2017-12-12 20:23 ` [PATCHSET v2] blk-mq: reimplement timeout handling Jens Axboe
2017-12-12 21:40 ` Tejun Heo
2017-12-20 23:41 ` Bart Van Assche
2017-12-21 0:08 ` tj
2017-12-21 1:00 ` Bart Van Assche
-- strict thread matches above, loose matches on Subject: below --
2017-12-09 19:25 [PATCHSET] " Tejun Heo
2017-12-09 19:25 ` [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme Tejun Heo
2017-12-12 10:09 ` Peter Zijlstra
2017-12-12 18:02 ` Tejun Heo
2017-12-12 10:10 ` Peter Zijlstra
2017-12-12 18:03 ` Tejun Heo
2017-12-12 11:56 ` Peter Zijlstra
2017-12-12 18:04 ` Tejun Heo
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