Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT

[Sebastian Andrzej Siewior]

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