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

[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
[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
[342431.665614]  do_syscall_64+0x5c/0xf0
[342431.665619]  ? __ct_user_enter+0x89/0x130
[342431.665623]  ? syscall_exit_to_user_mode+0x22/0x40
[342431.665625]  ? do_syscall_64+0x6b/0xf0
[342431.665627]  ? __ct_user_enter+0x89/0x130
[342431.665629]  entry_SYSCALL_64_after_hwframe+0x6e/0x76
[342431.665635] RIP: 0033:0x7f39856c757b
[342431.665666] RSP: 002b:00007ffd9541c488 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[342431.665670] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f39856c757b
[342431.665673] RDX: 0000000000000000 RSI: 0000000000001261 RDI: 0000000000000005
[342431.665674] RBP: 00007ffd9541c540 R08: 0000000000000003 R09: 006164732f766564
[342431.665676] R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffd9543ca68
[342431.665678] R13: 000055ea758a0708 R14: 000055ea759de338 R15: 00007f398586f000