Messages in this thread | | | Date | Mon, 9 Oct 2017 10:59:36 +0100 | From | Will Deacon <> | Subject | Re: [PATCH v2 0/5] Switch arm64 over to qrwlock |
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Hi Yury,
On Mon, Oct 09, 2017 at 12:30:52AM +0300, Yury Norov wrote: > On Fri, Oct 06, 2017 at 02:34:37PM +0100, Will Deacon wrote: > > This is version two of the patches I posted yesterday: > > > > http://lists.infradead.org/pipermail/linux-arm-kernel/2017-October/534666.html > > > > I'd normally leave it longer before posting again, but Peter had a good > > suggestion to rework the layout of the lock word, so I wanted to post a > > version that follows that approach. > > > > I've updated my branch if you're after the full patch stack: > > > > git://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git qrwlock > > > > As before, all comments (particularly related to testing and performance) > > welcome! > > > I tested your patches with locktorture and found measurable performance > regression. I also respin the patch of Jan Glauber [1], and I also > tried Jan's patch with patch 5 from this series. Numbers differ a lot > from my previous measurements, but since that I changed working > station and use qemu with the support of parallel threads. > Spinlock Read-RW lock Write-RW lock > Vanilla: 129804626 12340895 14716138 > This series: 113718002 10982159 13068934 > Jan patch: 117977108 11363462 13615449 > Jan patch + #5: 121483176 11696728 13618967 > > The bottomline of discussion [1] was that queued locks are more > effective when SoC has many CPUs. And 4 is not many. My measurement > was made on the 4-CPU machine, and it seems it confirms that. Does > it make sense to make queued locks default for many-CPU machines only?
Just to confirm, you're running this under qemu on an x86 host, using full AArch64 system emulation? If so, I really don't think we should base the merits of qrwlocks on arm64 around this type of configuration. Given that you work for a silicon vendor, could you try running on real arm64 hardware instead, please? My measurements on 6-core and 8-core systems look a lot better with qrwlock than what we currently have in mainline, and they also fix a real starvation issue reported by Jeremy [1].
I'd also add that lock fairness comes at a cost, so I'd expect a small drop in total throughput for some workloads. I encourage you to try passing different arguments to locktorture to see this in action. For example, on an 8-core machine:
# insmod ./locktorture.ko nwriters_stress=2 nreaders_stress=8 torture_type="rw_lock_irq" stat_interval=2
-rc3:
Writes: Total: 6612 Max/Min: 0/0 Fail: 0 Reads : Total: 1265230 Max/Min: 0/0 Fail: 0 Writes: Total: 6709 Max/Min: 0/0 Fail: 0 Reads : Total: 1916418 Max/Min: 0/0 Fail: 0 Writes: Total: 6725 Max/Min: 0/0 Fail: 0 Reads : Total: 5103727 Max/Min: 0/0 Fail: 0
notice how the writers are really struggling here (you only have to tweak a bit more and you get RCU stalls, lose interrupts etc).
With the qrwlock:
Writes: Total: 47962 Max/Min: 0/0 Fail: 0 Reads : Total: 277903 Max/Min: 0/0 Fail: 0 Writes: Total: 100151 Max/Min: 0/0 Fail: 0 Reads : Total: 525781 Max/Min: 0/0 Fail: 0 Writes: Total: 155284 Max/Min: 0/0 Fail: 0 Reads : Total: 767703 Max/Min: 0/0 Fail: 0
which is an awful lot better for maximum latency and fairness, despite the much lower reader count.
> There were 2 preparing patches in the series: > [PATCH 1/3] kernel/locking: #include <asm/spinlock.h> in qrwlock > and > [PATCH 2/3] asm-generic: don't #include <linux/atomic.h> in qspinlock_types.h > > 1st patch is not needed anymore because Babu Moger submitted similar patch that > is already in mainline: 9ab6055f95903 ("kernel/locking: Fix compile error with > qrwlock.c"). Could you revisit second patch?
Sorry, not sure what you're asking me to do here.
Will
[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2017-October/534299.html
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