Re: [dm-devel] dm-crypt: fix softlockup in dmcrypt_write

[Mikulas Patocka <mpatocka@redhat.com>]

On Mon, 6 Mar 2023, Bart Van Assche wrote:

> On 3/6/23 10:02, Mikulas Patocka wrote:
> > On Tue, 28 Feb 2023, yangerkun wrote:
> > > It's ok to fix the softlockup, but for async write encrypt,
> > > kcryptd_crypt_write_io_submit will add bio to write_tree, and once we
> > > call cond_resched before every kcryptd_io_write, the write performance
> > > may be poor while we meet a high cpu usage scene.
> > 
> > Hi
> > 
> > To fix this problem, find the PID of the process "dmcrypt_write" and
> > change its priority to -20, for example "renice -n -20 -p 34748".
> > 
> > This is the proper way how to fix it; locking up the process for one
> > second is not.
> > 
> > We used to have high-priority workqueues by default, but it caused audio
> > playback skipping, so we had to revert it - see
> > f612b2132db529feac4f965f28a1b9258ea7c22b.
> > 
> > Perhaps we should add an option to have high-priority kernel threads?
> 
> Would calling cond_resched() every n iterations instead of every iteration
> help? From mm/swapfile.c:
> 
> 			if (unlikely(--latency_ration < 0)) {
> 				cond_resched();
> 				latency_ration = LATENCY_LIMIT;
> 			}
> 
> Thanks,
> 
> Bart.

I think that if this helps, it is really a bug in the scheduler... It 
shouldn't switch tasks so often.

Mikulas
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