Re: [RFC PATCH v1 0/4] Introduce QPW for per-cpu operations

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Sep 06, 2024 at 06:19:08AM +0800, Hillf Danton wrote:
> On Tue, 23 Jul 2024 14:14:34 -0300 Marcelo Tosatti <mtosatti@redhat.com>
> > On Sat, Jun 22, 2024 at 12:58:08AM -0300, Leonardo Bras wrote:
> > > The problem:
> > > Some places in the kernel implement a parallel programming strategy
> > > consisting on local_locks() for most of the work, and some rare remote
> > > operations are scheduled on target cpu. This keeps cache bouncing low since
> > > cacheline tends to be mostly local, and avoids the cost of locks in non-RT
> > > kernels, even though the very few remote operations will be expensive due
> > > to scheduling overhead.
> > > 
> > > On the other hand, for RT workloads this can represent a problem: getting
> > > an important workload scheduled out to deal with remote requests is
> > > sure to introduce unexpected deadline misses.
> > 
> > Another hang with a busy polling workload (kernel update hangs on
> > grub2-probe):
> > 
> > [342431.665417] INFO: task grub2-probe:24484 blocked for more than 622 seconds.
> > [342431.665458]       Tainted: G        W      X  -------  ---  5.14.0-438.el9s.x86_64+rt #1
> > [342431.665488] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [342431.665515] task:grub2-probe     state:D stack:0     pid:24484 ppid:24455  flags:0x00004002
> > [342431.665523] Call Trace:
> > [342431.665525]  <TASK>
> > [342431.665527]  __schedule+0x22a/0x580
> > [342431.665537]  schedule+0x30/0x80
> > [342431.665539]  schedule_timeout+0x153/0x190
> > [342431.665543]  ? preempt_schedule_thunk+0x16/0x30
> > [342431.665548]  ? preempt_count_add+0x70/0xa0
> > [342431.665554]  __wait_for_common+0x8b/0x1c0
> > [342431.665557]  ? __pfx_schedule_timeout+0x10/0x10
> > [342431.665560]  __flush_work.isra.0+0x15b/0x220
> 
> The fresh new flush_percpu_work() is nop with CONFIG_PREEMPT_RT enabled, why
> are you testing it with 5.14.0-438.el9s.x86_64+rt instead of mainline? Or what
> are you testing?

I am demonstrating a type of bug that can happen without Leo's patch.

> BTW the hang fails to show the unexpected deadline misses.

Yes, because in this case the realtime app with FIFO priority never
stops running, therefore grub2-probe hangs and is unable to execute:

> > [342431.665417] INFO: task grub2-probe:24484 blocked for more than 622 seconds
> 
> > [342431.665565]  ? __pfx_wq_barrier_func+0x10/0x10
> > [342431.665570]  __lru_add_drain_all+0x17d/0x220
> > [342431.665576]  invalidate_bdev+0x28/0x40
> > [342431.665583]  blkdev_common_ioctl+0x714/0xa30
> > [342431.665588]  ? bucket_table_alloc.isra.0+0x1/0x150
> > [342431.665593]  ? cp_new_stat+0xbb/0x180
> > [342431.665599]  blkdev_ioctl+0x112/0x270
> > [342431.665603]  ? security_file_ioctl+0x2f/0x50
> > [342431.665609]  __x64_sys_ioctl+0x87/0xc0

Does that make sense now?

Thanks!