Re: [f2fs-dev] [PATCH] f2fs: handle decompress only post processing in softirq

[Gao Xiang]

Hi all,

On Mon, Jun 13, 2022 at 10:38:25PM -0700, Eric Biggers wrote:
> [+Cc Nathan Huckleberry who is looking into a similar problem in dm-verity]
> 
> On Mon, Jun 13, 2022 at 08:56:12AM -0700, Daeho Jeong wrote:
> > From: Daeho Jeong <daehojeong@xxxxxxxxxx>
> > 
> > Now decompression is being handled in workqueue and it makes read I/O
> > latency non-deterministic, because of the non-deterministic scheduling
> > nature of workqueues. So, I made it handled in softirq context only if
> > possible.
> > 
> > Signed-off-by: Daeho Jeong <daehojeong@xxxxxxxxxx>

...

> 
> One question: is this (the bio endio callback) actually guaranteed to be
> executed from a softirq?  If you look at dm-crypt's support for workqueue-less
> decryption, for example, it explicitly checks 'in_hardirq() || irqs_disabled()'
> and schedules a tasklet if either of those is the case.
> 
> - Eric
> 

Some my own previous thoughts about this strategy:

 - If we allocate all memory and map these before I/Os, all inflight I/Os
   will keep such temporary pages all the time until decompression is
   finished. In contrast, if we allocate or reuse such pages just before
   decompression, it would minimize the memory footprints.

   I think it will impact the memory numbers at least on the very
   low-ended devices with bslow storage. (I've seen f2fs has some big
   mempool already)

 - Many compression algorithms are not suitable in the softirq contexts,
   also I vaguely remembered if softirq context lasts for > 2ms, it will
   push into ksoftirqd instead so it's actually another process context.
   And it may delay other important interrupt handling.

 - Go back to the non-deterministic scheduling of workqueues. I guess it
   may be just due to scheduling punishment due to a lot of CPU consuming
   due to decompression before so the priority becomes low, but that is
   just a pure guess. May be we need to use RT scheduling policy instead.

   At least with WQ_HIGHPRI for dm-verity at least, but I don't find
   WQ_HIGHPRI mark for dm-verity.

Thanks,
Gao Xiang