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Date: Thu, 28 Jul 2016 03:10:31 +0000 (UTC) From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> To: Boqun Feng <boqun.feng@...il.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Russell King <linux@....linux.org.uk>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, linux-kernel <linux-kernel@...r.kernel.org>, linux-api <linux-api@...r.kernel.org>, Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>, Peter Zijlstra <peterz@...radead.org>, Andy Lutomirski <luto@...capital.net>, Andi Kleen <andi@...stfloor.org>, Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Josh Triplett <josh@...htriplett.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, Michael Kerrisk <mtk.manpages@...il.com>, Michael Ellerman <mpe@...erman.id.au>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org> Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call ----- On Jul 27, 2016, at 11:03 AM, Boqun Feng boqun.feng@...il.com wrote: > Hi Mathieu, > > On Thu, Jul 21, 2016 at 05:14:16PM -0400, Mathieu Desnoyers wrote: >> Expose a new system call allowing each thread to register one userspace >> memory area to be used as an ABI between kernel and user-space for two >> purposes: user-space restartable sequences and quick access to read the >> current CPU number value from user-space. >> >> * Restartable sequences (per-cpu atomics) >> >> The restartable critical sections (percpu atomics) work has been started >> by Paul Turner and Andrew Hunter. It lets the kernel handle restart of >> critical sections. [1] [2] The re-implementation proposed here brings a >> few simplifications to the ABI which facilitates porting to other > > Agreed ;-) > >> architectures and speeds up the user-space fast path. A locking-based >> fall-back, purely implemented in user-space, is proposed here to deal >> with debugger single-stepping. This fallback interacts with rseq_start() >> and rseq_finish(), which force retries in response to concurrent >> lock-based activity. >> > > So I have enabled this on powerpc, thanks to your nice work to make > things easy for porting ;-) > > A patchset will follow in-reply-to this email, which includes patches > enabling this on powerpc and a patch that improves the portability of > the selftests, which I think it's not necessary to be a standalone > patch, so it's OK to be merged into your patch #7. > > I did some tests on 64bit little/big endian pSeries(guest) kernel with > selftest cases(64bit LE selftest on 64bit LE kernel, 64/32bit BE > selftest on 64bit BE kernel), things seemingly went well ;-) > > Here are some benchmark results I got on a little endian guest with 64 > VCPUs: > > Benchmarking various approaches for reading the current CPU number: > > Power8 PSeries Guest(64 VCPUs, the host has 16 cores, 128 hardware > threads): > > - Baseline (empty loop): 1.56 ns > - Read CPU from rseq cpu_id: 1.56 ns > - Read CPU from rseq cpu_id (lazy register): 2.08 ns > - glibc 2.23-0ubuntu3 getcpu: 7.72 ns > - getcpu system call: 91.80 ns > > > Benchmarking various approaches for counter increment: > > Power8 PSeries KVM Guest(64 VCPUs, the host has 16 cores, 128 hardware > threads): > > Counter increment speed (ns/increment) > 1 thread 2 threads 4 threads 8 threads 16 threads 32 threads > global increment (baseline) 6.5 N/A N/A N/A > N/A N/A > percpu rseq increment 6.9 6.9 7.2 7.3 > 15.4 35.5 > percpu rseq spinlock 19.0 18.9 19.4 19.4 > 35.5 71.8 > global atomic increment 25.8 111.0 261.0 905.2 > 2319.5 4170.5 (__sync_add_and_fetch_4) > global atomic CAS 26.2 119.0 341.6 1183.0 > 3951.3 9312.5 (__sync_val_compare_and_swap_4) > global pthread mutex 40.0 238.1 644.0 2052.2 > 4272.5 8612.2 > > > I surely need to run more tests for my patches in different > environments, and will try to adjust the patchset according to whatever > change you make(e.g. rseq_finish2) in the future. I'm very glad to see it brings speedup on powerpc too! I plan minor changes following the feedback I already got. I'll surely grab your updated benchmark numbers into my changelog when I stop hiding in RFC. ;) Thanks, Mathieu > > (Add PPC maintainers in Cc) > > Regards, > Boqun > >> Here are benchmarks of counter increment in various scenarios compared >> to restartable sequences: >> >> ARMv7 Processor rev 4 (v7l) >> Machine model: Cubietruck >> >> Counter increment speed (ns/increment) >> 1 thread 2 threads >> global increment (baseline) 6 N/A >> percpu rseq increment 50 52 >> percpu rseq spinlock 94 94 >> global atomic increment 48 74 (__sync_add_and_fetch_4) >> global atomic CAS 50 172 (__sync_val_compare_and_swap_4) >> global pthread mutex 148 862 >> >> ARMv7 Processor rev 10 (v7l) >> Machine model: Wandboard >> >> Counter increment speed (ns/increment) >> 1 thread 4 threads >> global increment (baseline) 7 N/A >> percpu rseq increment 50 50 >> percpu rseq spinlock 82 84 >> global atomic increment 44 262 (__sync_add_and_fetch_4) >> global atomic CAS 46 316 (__sync_val_compare_and_swap_4) >> global pthread mutex 146 1400 >> >> x86-64 Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz: >> >> Counter increment speed (ns/increment) >> 1 thread 8 threads >> global increment (baseline) 3.0 N/A >> percpu rseq increment 3.6 3.8 >> percpu rseq spinlock 5.6 6.2 >> global LOCK; inc 8.0 166.4 >> global LOCK; cmpxchg 13.4 435.2 >> global pthread mutex 25.2 1363.6 >> >> * Reading the current CPU number >> >> Speeding up reading the current CPU number on which the caller thread is >> running is done by keeping the current CPU number up do date within the >> cpu_id field of the memory area registered by the thread. This is done >> by making scheduler migration set the TIF_NOTIFY_RESUME flag on the >> current thread. Upon return to user-space, a notify-resume handler >> updates the current CPU value within the registered user-space memory >> area. User-space can then read the current CPU number directly from >> memory. >> >> Keeping the current cpu id in a memory area shared between kernel and >> user-space is an improvement over current mechanisms available to read >> the current CPU number, which has the following benefits over >> alternative approaches: >> >> - 35x speedup on ARM vs system call through glibc >> - 20x speedup on x86 compared to calling glibc, which calls vdso >> executing a "lsl" instruction, >> - 14x speedup on x86 compared to inlined "lsl" instruction, >> - Unlike vdso approaches, this cpu_id value can be read from an inline >> assembly, which makes it a useful building block for restartable >> sequences. >> - The approach of reading the cpu id through memory mapping shared >> between kernel and user-space is portable (e.g. ARM), which is not the >> case for the lsl-based x86 vdso. >> >> On x86, yet another possible approach would be to use the gs segment >> selector to point to user-space per-cpu data. This approach performs >> similarly to the cpu id cache, but it has two disadvantages: it is >> not portable, and it is incompatible with existing applications already >> using the gs segment selector for other purposes. >> >> Benchmarking various approaches for reading the current CPU number: >> >> ARMv7 Processor rev 4 (v7l) >> Machine model: Cubietruck >> - Baseline (empty loop): 8.4 ns >> - Read CPU from rseq cpu_id: 16.7 ns >> - Read CPU from rseq cpu_id (lazy register): 19.8 ns >> - glibc 2.19-0ubuntu6.6 getcpu: 301.8 ns >> - getcpu system call: 234.9 ns >> >> x86-64 Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz: >> - Baseline (empty loop): 0.8 ns >> - Read CPU from rseq cpu_id: 0.8 ns >> - Read CPU from rseq cpu_id (lazy register): 0.8 ns >> - Read using gs segment selector: 0.8 ns >> - "lsl" inline assembly: 13.0 ns >> - glibc 2.19-0ubuntu6 getcpu: 16.6 ns >> - getcpu system call: 53.9 ns >> >> - Speed >> >> Running 10 runs of hackbench -l 100000 seems to indicate, contrary to >> expectations, that enabling CONFIG_RSEQ slightly accelerates the >> scheduler: >> >> Configuration: 2 sockets * 8-core Intel(R) Xeon(R) CPU E5-2630 v3 @ >> 2.40GHz (directly on hardware, hyperthreading disabled in BIOS, energy >> saving disabled in BIOS, turboboost disabled in BIOS, cpuidle.off=1 >> kernel parameter), with a Linux v4.6 defconfig+localyesconfig, >> restartable sequences series applied. >> > > [snip] -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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