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 <daehoje...@google.com> > > > > 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 <daehoje...@google.com>
... > > 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 _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
