Re: [PATCH -next 1/8] md/raid10: prevent soft lockup while flush writes

[Song Liu]

On Thu, Apr 20, 2023 at 4:31 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Currently, there is no limit for raid1/raid10 plugged bio. While flushing
> writes, raid1 has cond_resched() while raid10 doesn't, and too many
> writes can cause soft lockup.
>
> Follow up soft lockup can be triggered easily with writeback test for
> raid10 with ramdisks:
>
> watchdog: BUG: soft lockup - CPU#10 stuck for 27s! [md0_raid10:1293]
> Call Trace:
>  <TASK>
>  call_rcu+0x16/0x20
>  put_object+0x41/0x80
>  __delete_object+0x50/0x90
>  delete_object_full+0x2b/0x40
>  kmemleak_free+0x46/0xa0
>  slab_free_freelist_hook.constprop.0+0xed/0x1a0
>  kmem_cache_free+0xfd/0x300
>  mempool_free_slab+0x1f/0x30
>  mempool_free+0x3a/0x100
>  bio_free+0x59/0x80
>  bio_put+0xcf/0x2c0
>  free_r10bio+0xbf/0xf0
>  raid_end_bio_io+0x78/0xb0
>  one_write_done+0x8a/0xa0
>  raid10_end_write_request+0x1b4/0x430
>  bio_endio+0x175/0x320
>  brd_submit_bio+0x3b9/0x9b7 [brd]
>  __submit_bio+0x69/0xe0
>  submit_bio_noacct_nocheck+0x1e6/0x5a0
>  submit_bio_noacct+0x38c/0x7e0
>  flush_pending_writes+0xf0/0x240
>  raid10d+0xac/0x1ed0
>
> This patch fix the problem by adding cond_resched() to raid10 like what
> raid1 did.

nit: per submitting-patches.rst:

Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour.

>
> Note that unlimited plugged bio still need to be optimized because in
> the case of writeback lots of dirty pages, this will take lots of memory
> and io latecy is quite bad.

typo: latency.

>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  drivers/md/raid10.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 6590aa49598c..a116b7c9d9f3 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -921,6 +921,7 @@ static void flush_pending_writes(struct r10conf *conf)
>                         else
>                                 submit_bio_noacct(bio);
>                         bio = next;
> +                       cond_resched();
>                 }
>                 blk_finish_plug(&plug);
>         } else
> @@ -1140,6 +1141,7 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
>                 else
>                         submit_bio_noacct(bio);
>                 bio = next;
> +               cond_resched();
>         }
>         kfree(plug);
>  }
> --
> 2.39.2
>