From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Christoph Hellwig <hch@infradead.org>
Cc: David Runge <dave@sleepmap.de>,
linux-rt-users@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Daniel Wagner <dwagner@suse.de>
Subject: Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT
Date: Tue, 27 Oct 2020 11:11:02 +0100 [thread overview]
Message-ID: <20201027101102.cvczdb3mkvtoguo5@linutronix.de> (raw)
In-Reply-To: <20201027092606.GA20805@infradead.org>
On 2020-10-27 09:26:06 [+0000], Christoph Hellwig wrote:
> On Fri, Oct 23, 2020 at 03:52:19PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2020-10-23 12:21:30 [+0100], Christoph Hellwig wrote:
> > > > - if (!IS_ENABLED(CONFIG_SMP) ||
> > > > + if (!IS_ENABLED(CONFIG_SMP) || IS_ENABLED(CONFIG_PREEMPT_RT) ||
> > > > !test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags))
> > >
> > > This needs a big fat comment explaining your rationale. And probably
> > > a separate if statement to make it obvious as well.
> >
> > Okay.
> > How much difference does it make between completing in-softirq vs
> > in-IPI?
>
> For normal non-RT builds? This introduces another context switch, which
> for the latencies we are aiming for is noticable.
There should be no context switch. The pending softirq should be
executed on irq_exit() from that IPI, that is
irq_exit()
-> __irq_exit_rcu()
-> invoke_softirq()
-> __do_softirq() || do_softirq_own_stack()
unlike with the command line switch `threadirqs' enabled,
invoke_softirq() woukd wakeup the `ksoftirqd' thread which would involve
a context switch.
> > I'm asking because acquiring a spinlock_t in an IPI shouldn't be
> > done (as per Documentation/locking/locktypes.rst). We don't have
> > anything in lockdep that will complain here on !RT and we the above we
> > avoid the case on RT.
>
> At least for NVMe we aren't taking locks, but with the number of drivers
Right. I found this David Runge's log:
|BUG: scheduling while atomic: swapper/19/0/0x00010002
|CPU: 19 PID: 0 Comm: swapper/19 Not tainted 5.9.1-rt18-1-rt #1
|Hardware name: System manufacturer System Product Name/Pro WS X570-ACE, BIOS 1302 01/20/2020
|Call Trace:
| <IRQ>
| dump_stack+0x6b/0x88
| __schedule_bug.cold+0x89/0x97
| __schedule+0x6a4/0xa10
| preempt_schedule_lock+0x23/0x40
| rt_spin_lock_slowlock_locked+0x117/0x2c0
| rt_spin_lock_slowlock+0x58/0x80
| rt_spin_lock+0x2a/0x40
| test_clear_page_writeback+0xcd/0x310
| end_page_writeback+0x43/0x70
| end_bio_extent_buffer_writepage+0xb2/0x100 [btrfs]
| btrfs_end_bio+0x83/0x140 [btrfs]
| clone_endio+0x84/0x1f0 [dm_mod]
| blk_update_request+0x254/0x470
| blk_mq_end_request+0x1c/0x130
| flush_smp_call_function_queue+0xd5/0x1a0
| __sysvec_call_function_single+0x36/0x150
| asm_call_irq_on_stack+0x12/0x20
| </IRQ>
so the NVME driver isn't taking any locks but lock_page_memcg() (and
xa_lock_irqsave()) in test_clear_page_writeback() is.
Sebastian
next prev parent reply other threads:[~2020-10-27 10:11 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-21 17:50 5.9.1-rt18: issues with Firewire card on AMD hardware David Runge
2020-10-23 11:04 ` [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT Sebastian Andrzej Siewior
2020-10-23 11:21 ` Christoph Hellwig
2020-10-23 13:52 ` Sebastian Andrzej Siewior
2020-10-27 9:26 ` Christoph Hellwig
2020-10-27 10:11 ` Sebastian Andrzej Siewior [this message]
2020-10-27 16:07 ` Christoph Hellwig
2020-10-27 17:05 ` Thomas Gleixner
2020-10-27 17:23 ` Christoph Hellwig
2020-10-27 17:59 ` Sebastian Andrzej Siewior
2020-10-27 20:58 ` Sebastian Andrzej Siewior
2020-10-28 6:56 ` Christoph Hellwig
2020-10-28 14:12 ` [PATCH 1/3] blk-mq: Don't complete on a remote CPU in force threaded mode Sebastian Andrzej Siewior
2020-10-28 14:12 ` [PATCH 2/3] blk-mq: Always complete remote completions requests in softirq Sebastian Andrzej Siewior
2020-10-28 14:12 ` [PATCH 3/3] blk-mq: Use llist_head for blk_cpu_done Sebastian Andrzej Siewior
2020-10-28 14:44 ` Christoph Hellwig
2020-10-28 14:47 ` Sebastian Andrzej Siewior
2020-10-29 13:12 ` Sebastian Andrzej Siewior
2020-10-29 14:05 ` Christoph Hellwig
2020-10-29 14:56 ` Sebastian Andrzej Siewior
2020-10-29 14:57 ` Christoph Hellwig
2020-10-29 20:03 ` Sagi Grimberg
2020-10-29 21:01 ` Sebastian Andrzej Siewior
2020-10-29 21:07 ` Sagi Grimberg
2020-10-31 10:41 ` Sebastian Andrzej Siewior
2020-10-31 15:00 ` Jens Axboe
2020-10-31 15:01 ` Jens Axboe
2020-10-31 18:09 ` Christoph Hellwig
2020-11-02 9:55 ` Sebastian Andrzej Siewior
2020-11-02 18:12 ` Christoph Hellwig
2020-11-04 19:15 ` Sagi Grimberg
2020-11-06 15:23 ` Sebastian Andrzej Siewior
2020-10-28 10:04 ` [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT Peter Zijlstra
2020-10-26 0:37 ` 5.9.1-rt18: issues with Firewire card on AMD hardware David Runge
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20201027101102.cvczdb3mkvtoguo5@linutronix.de \
--to=bigeasy@linutronix.de \
--cc=axboe@kernel.dk \
--cc=dave@sleepmap.de \
--cc=dwagner@suse.de \
--cc=hch@infradead.org \
--cc=linux-block@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-rt-users@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).