On Thu, 2021-05-27 at 08:50 +0200, Greg Kroah-Hartman wrote: > On Thu, May 27, 2021 at 09:23:06AM +0800, Ian Kent wrote: > > On Mon, 2021-05-17 at 09:32 +0800, Ian Kent wrote: > > > On Fri, 2021-05-14 at 10:34 +0800, Fox Chen wrote: > > > > On Fri, May 14, 2021 at 9:34 AM Ian Kent <raven@xxxxxxxxxx> > > > > wrote: > > > > > > > > > > On Thu, 2021-05-13 at 23:37 +0800, Fox Chen wrote: > > > > > > Hi Ian > > > > > > > > > > > > On Thu, May 13, 2021 at 10:10 PM Ian Kent > > > > > > <raven@xxxxxxxxxx> > > > > > > wrote: > > > > > > > > > > > > > > On Wed, 2021-05-12 at 16:54 +0800, Fox Chen wrote: > > > > > > > > On Wed, May 12, 2021 at 4:47 PM Fox Chen > > > > > > > > <foxhlchen@xxxxxxxxx> > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > > > > I ran it on my benchmark ( > > > > > > > > > https://github.com/foxhlchen/sysfs_benchmark). > > > > > > > > > > > > > > > > > > machine: aws c5 (Intel Xeon with 96 logical cores) > > > > > > > > > kernel: v5.12 > > > > > > > > > benchmark: create 96 threads and bind them to each > > > > > > > > > core > > > > > > > > > then > > > > > > > > > run > > > > > > > > > open+read+close on a sysfs file simultaneously for > > > > > > > > > 1000 > > > > > > > > > times. > > > > > > > > > result: > > > > > > > > > Without the patchset, an open+read+close operation > > > > > > > > > takes > > > > > > > > > 550- > > > > > > > > > 570 > > > > > > > > > us, > > > > > > > > > perf shows significant time(>40%) spending on > > > > > > > > > mutex_lock. > > > > > > > > > After applying it, it takes 410-440 us for that > > > > > > > > > operation > > > > > > > > > and > > > > > > > > > perf > > > > > > > > > shows only ~4% time on mutex_lock. > > > > > > > > > > > > > > > > > > It's weird, I don't see a huge performance boost > > > > > > > > > compared > > > > > > > > > to > > > > > > > > > v2, > > > > > > > > > even > > > > > > > > > > > > > > > > I meant I don't see a huge performance boost here and > > > > > > > > it's > > > > > > > > way > > > > > > > > worse > > > > > > > > than v2. > > > > > > > > IIRC, for v2 fastest one only takes 40us > > > > > > > > > > > > > > Thanks Fox, > > > > > > > > > > > > > > I'll have a look at those reports but this is puzzling. > > > > > > > > > > > > > > Perhaps the added overhead of the check if an update is > > > > > > > needed is taking more than expected and more than just > > > > > > > taking the lock and being done with it. Then there's > > > > > > > the v2 series ... I'll see if I can dig out your reports > > > > > > > on those too. > > > > > > > > > > > > Apologies, I was mistaken, it's compared to V3, not V2. > > > > > > The > > > > > > previous > > > > > > benchmark report is here. > > > > > > https://lore.kernel.org/linux-fsdevel/CAC2o3DKNc=sL2n8291Dpiyb0bRHaX=nd33ogvO_LkJqpBj-YmA@xxxxxxxxxxxxxx/ > > > > > > > > > > Are all these tests using a single file name in the > > > > > open/read/close > > > > > loop? > > > > > > > > Yes, because It's easy to implement yet enough to trigger the > > > > mutex_lock. > > > > > > > > And you are right It's not a real-life pattern, but on the > > > > bright > > > > side, it proves there is no original mutex_lock problem > > > > anymore. :) > > > > > > I've been looking at your reports and they are quite interesting. > > > > > > > > > > > > That being the case the per-object inode lock will behave > > > > > like a > > > > > mutex and once contention occurs any speed benefits of a > > > > > spinlock > > > > > over a mutex (or rwsem) will disappear. > > > > > > > > > > In this case changing from a write lock to a read lock in > > > > > those > > > > > functions and adding the inode mutex will do nothing but add > > > > > the > > > > > overhead of taking the read lock. And similarly adding the > > > > > update > > > > > check function also just adds overhead and, as we see, once > > > > > contention starts it has a cumulative effect that's often not > > > > > linear. > > > > > > > > > > The whole idea of a read lock/per-object spin lock was to > > > > > reduce > > > > > the possibility of contention for paths other than the same > > > > > path > > > > > while not impacting same path accesses too much for an > > > > > overall > > > > > gain. Based on this I'm thinking the update check function is > > > > > probably not worth keeping, it just adds unnecessary churn > > > > > and > > > > > has a negative impact for same file contention access > > > > > patterns. > > > > > > The reports indicate (to me anyway) that the slowdown isn't > > > due to kernfs. It looks more like kernfs is now putting pressure > > > on the VFS, mostly on the file table lock but it looks like > > > there's a mild amount of contention on a few other locks as well > > > now. > > > > > > That's a whole different problem and those file table handling > > > functions don't appear to have any obvious problems so they are > > > doing what they have to do and that can't be avoided. > > > > > > That's definitely out of scope for these changes. > > > > > > And, as you'd expect, once any appreciable amount of contention > > > happens our measurements go out the window, certainly with > > > respect to kernfs. > > > > > > It also doesn't change my option that checking if an inode > > > attribute update is needed in kernfs isn't useful since, IIUC > > > that file table lock contention would result even if you were > > > using different paths. > > > > > > So I'll drop that patch from the series. > > > > It will probably not make any difference, but for completeness > > and after some thought, I felt I should post what I think is > > happening with the benchmark. > > > > The benchmark application runs a pthreads thread for each CPU > > and goes into a tight open/read/close loop to demonstrate the > > contention that can occur on the kernfs mutex as the number of > > CPUs grows. > > > > But that tight open/read/close loop causes contention on the VFS > > file table because the pthreads threads share the process file > > table. > > > > So the poor performance is actually evidence the last patch is > > doing what it's meant to do rather than evidence of a regression > > with the series. > > > > The benchmark is putting pressure on the process file table on > > some hardware configurations but those critical sections are small > > and there's really nothing obvious that can be done to improve the > > file table locking. > > > > It is however important to remember that the benckmark application > > access pattern is not a normal access pattern by a long way. > > > > So I don't see the need for a new revision of the series with the > > last patch dropped. > > > > If there's a change of heart and the series was to be merged I'll > > leave whether to include this last patch to the discretion of the > > maintainer as the bulk of the improvement comes from the earlier > > patches anyway. > > Can you please resubmit the series, it is long-gone from my review > queue. Sure, will do as soon as I can. Thanks Ian
