Messages in this thread | | | Date | Thu, 13 Jun 2019 04:27:56 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 3/7] perf: arm64: Use rseq to test userspace access to pmu counters |
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----- On Jun 13, 2019, at 9:10 AM, Raphael Gault raphael.gault@arm.com wrote:
> Hi Mathieu, > > On 6/11/19 8:33 PM, Mathieu Desnoyers wrote: >> ----- On Jun 11, 2019, at 6:57 PM, Mark Rutland mark.rutland@arm.com wrote: >> >>> Hi Arnaldo, >>> >>> On Tue, Jun 11, 2019 at 11:33:46AM -0300, Arnaldo Carvalho de Melo wrote: >>>> Em Tue, Jun 11, 2019 at 01:53:11PM +0100, Raphael Gault escreveu: >>>>> Add an extra test to check userspace access to pmu hardware counters. >>>>> This test doesn't rely on the seqlock as a synchronisation mechanism but >>>>> instead uses the restartable sequences to make sure that the thread is >>>>> not interrupted when reading the index of the counter and the associated >>>>> pmu register. >>>>> >>>>> In addition to reading the pmu counters, this test is run several time >>>>> in order to measure the ratio of failures: >>>>> I ran this test on the Juno development platform, which is big.LITTLE >>>>> with 4 Cortex A53 and 2 Cortex A57. The results vary quite a lot >>>>> (running it with 100 tests is not so long and I did it several times). >>>>> I ran it once with 10000 iterations: >>>>> `runs: 10000, abort: 62.53%, zero: 34.93%, success: 2.54%` >>>>> >>>>> Signed-off-by: Raphael Gault <raphael.gault@arm.com> >>>>> --- >>>>> tools/perf/arch/arm64/include/arch-tests.h | 5 +- >>>>> tools/perf/arch/arm64/include/rseq-arm64.h | 220 ++++++++++++++++++ >>>> >>>> So, I applied the first patch in this series, but could you please break >>>> this patch into at least two, one introducing the facility >>>> (include/rseq*) and the second adding the test? >>>> >>>> We try to enforce this kind of granularity as down the line we may want >>>> to revert one part while the other already has other uses and thus >>>> wouldn't allow a straight revert. >>>> >>>> Also, can this go to tools/arch/ instead? Is this really perf specific? >>>> Isn't there any arch/arm64/include files for the kernel that we could >>>> mirror and have it checked for drift in tools/perf/check-headers.sh? >>> >>> The rseq bits aren't strictly perf specific, and I think the existing >>> bits under tools/testing/selftests/rseq/ could be factored out to common >>> locations under tools/include/ and tools/arch/*/include/. >> >> Hi Mark, >> >> Thanks for CCing me! >> >> Or into a stand-alone librseq project: >> >> https://github.com/compudj/librseq (currently a development branch in >> my own github) >> >> I don't see why this user-space code should sit in the kernel tree. >> It is not tooling-specific. >> >>> >>> From a scan, those already duplicate barriers and other helpers which >>> already have definitions under tools/, which seems unfortunate. :/ >>> >>> Comments below are for Raphael and Matthieu. >>> >>> [...] >>> >>>>> +static u64 noinline mmap_read_self(void *addr, int cpu) >>>>> +{ >>>>> + struct perf_event_mmap_page *pc = addr; >>>>> + u32 idx = 0; >>>>> + u64 count = 0; >>>>> + >>>>> + asm volatile goto( >>>>> + RSEQ_ASM_DEFINE_TABLE(0, 1f, 2f, 3f) >>>>> + "nop\n" >>>>> + RSEQ_ASM_STORE_RSEQ_CS(1, 0b, rseq_cs) >>>>> + RSEQ_ASM_CMP_CPU_ID(cpu_id, current_cpu_id, 3f) >>>>> + RSEQ_ASM_OP_R_LOAD(pc_idx) >>>>> + RSEQ_ASM_OP_R_AND(0xFF) >>>>> + RSEQ_ASM_OP_R_STORE(idx) >>>>> + RSEQ_ASM_OP_R_SUB(0x1) >>>>> + RSEQ_ASM_CMP_CPU_ID(cpu_id, current_cpu_id, 3f) >>>>> + "msr pmselr_el0, " RSEQ_ASM_TMP_REG "\n" >>>>> + "isb\n" >>>>> + RSEQ_ASM_CMP_CPU_ID(cpu_id, current_cpu_id, 3f) >> >> I really don't understand why the cpu_id needs to be compared 3 times >> here (?!?) >> >> Explicit comparison of the cpu_id within the rseq critical section >> should be done _once_. >> > > I understand and that's what I thought as well but I got confused with a > comment in (src)/include/uapi/linux/rseq.h which states: > > This CPU number value should always be compared > > against the value of the cpu_id field before performing a rseq > > commit or returning a value read from a data structure indexed > > using the cpu_id_start value. > > I'll remove the unnecessary testing.
It needs to be compared at least once, yes. But once is enough.
Thanks,
Mathieu
> > >> If the kernel happens to preempt and migrate the thread while in the >> critical section, it's the kernel's job to move user-space execution >> to the abort handler. >> > [...] > > Thanks, > > -- > Raphael Gault
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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