Messages in this thread | | | Date | Tue, 22 Mar 2022 13:37:24 -0400 | Subject | Re: [GIT PULL] f2fs for 5.18 | From | Waiman Long <> |
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On 3/22/22 13:22, Linus Torvalds wrote: > On Mon, Mar 21, 2022 at 1:39 PM Jaegeuk Kim <jaegeuk@kernel.org> wrote: >> In this cycle, f2fs has some performance improvements for Android workloads such >> as using read-unfair rwsems [...] > I've pulled this, but that read-unfair rwsem code looks incredibly > dodgy. Doing your own locking is always a bad sign, and it ahs > traditionally come back to bite us pretty much every time. At least it > uses real lock primitives, just in a really odd way. > > The whole notion of making an rwsem unfair to readers sounds really > really odd. I mean, the whole and only _point_ of an rwsem is to > allow concurrent readers, and traditionally if it's unfair it's unfair > to _writers_ because that tends to be better for throughput (but > unfairness can cause horrible latency). > > So it smells like there's something bad going on in f2fs. > > That said, I'm adding Waiman to the cc here in case he would have > ideas at least for a cleaner interface. Our rw_semaphores are > explicitly trying to be fair, because unfairness (the other way) was > such a big problem. > > I'm wondering it the optimistic read lock stealing is bothering f2fs?
I don't believe it is the optimistic read lock stealing code that is bothering f2fs.
AFAICS, the read-unfair rwsem code is created to resolve a potential lock starvation problem that they found on linux-5.10.y stable tree. I believe I have fixed that in the v5.11 kernel, see commit 2f06f702925 ("locking/rwsem: Prevent potential lock starvation"). However that commit is not in the stable tree. In fact, I have moved forward and taken out reader optimistic spinning but added just optimistic lock stealing instead. I believe the problem would have solved by including that patch series in their build. I haven't gotten any response as to whether they had tested this or not.
Apparently they prefer to upstream this stop-gap solution.
Cheers, Longman
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