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Date: Thu, 10 Aug 2023 16:27:11 +0100
From: Marc Zyngier <maz@...nel.org>
To: Huang Shijie <shijie@...amperecomputing.com>
Cc: oliver.upton@...ux.dev, james.morse@....com,
suzuki.poulose@....com, yuzenghui@...wei.com,
catalin.marinas@....com, will@...nel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
linux-kernel@...r.kernel.org, patches@...erecomputing.com,
zwang@...erecomputing.com, Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2] KVM/arm64: reconfigurate the event filters for guest context
Huang,
Please make sure you add everyone who commented on v1 (I've Cc'd Mark
so that he can shime need as needed).
On Thu, 10 Aug 2023 08:29:06 +0100,
Huang Shijie <shijie@...amperecomputing.com> wrote:
>
> 1.) Background.
> 1.1) In arm64, start a guest with Qemu which is running as a VMM of KVM,
> and bind the guest to core 33 and run program "a" in guest.
> The code of "a" shows below:
> ----------------------------------------------------------
> #include <stdio.h>
>
> int main()
> {
> unsigned long i = 0;
>
> for (;;) {
> i++;
> }
>
> printf("i:%ld\n", i);
> return 0;
> }
> ----------------------------------------------------------
>
> 1.2) Use the following perf command in host:
> #perf stat -e cycles:G,cycles:H -C 33 -I 1000 sleep 1
> # time counts unit events
> 1.000817400 3,299,471,572 cycles:G
> 1.000817400 3,240,586 cycles:H
>
> This result is correct, my cpu's frequency is 3.3G.
>
> 1.3) Use the following perf command in host:
> #perf stat -e cycles:G,cycles:H -C 33 -d -d -I 1000 sleep 1
> time counts unit events
> 1.000831480 153,634,097 cycles:G (70.03%)
> 1.000831480 3,147,940,599 cycles:H (70.03%)
> 1.000831480 1,143,598,527 L1-dcache-loads (70.03%)
> 1.000831480 9,986 L1-dcache-load-misses # 0.00% of all L1-dcache accesses (70.03%)
> 1.000831480 <not supported> LLC-loads
> 1.000831480 <not supported> LLC-load-misses
> 1.000831480 580,887,696 L1-icache-loads (70.03%)
> 1.000831480 77,855 L1-icache-load-misses # 0.01% of all L1-icache accesses (70.03%)
> 1.000831480 6,112,224,612 dTLB-loads (70.03%)
> 1.000831480 16,222 dTLB-load-misses # 0.00% of all dTLB cache accesses (69.94%)
> 1.000831480 590,015,996 iTLB-loads (59.95%)
> 1.000831480 505 iTLB-load-misses # 0.00% of all iTLB cache accesses (59.95%)
>
> This result is wrong. The "cycle:G" should be nearly 3.3G.
>
> 2.) Root cause.
> There is only 7 counters in my arm64 platform:
> (one cycle counter) + (6 normal counters)
>
> In 1.3 above, we will use 10 event counters.
> Since we only have 7 counters, the perf core will trigger
> multiplexing in hrtimer:
> perf_mux_hrtimer_restart() --> perf_rotate_context().
>
> If the hrtimer occurs when the host is running, it's fine.
> If the hrtimer occurs when the guest is running,
> the perf_rotate_context() will program the PMU with filters for
> host context. The KVM does not have a chance to restore
> PMU registers with kvm_vcpu_pmu_restore_guest().
> The PMU does not work correctly, so we got wrong result.
>
> 3.) About this patch.
> Make a KVM_REQ_RELOAD_PMU request before reentering the
> guest. The request will call kvm_vcpu_pmu_restore_guest()
> to reconfigurate the filters for guest context.
>
> 4.) Test result of this patch:
> #perf stat -e cycles:G,cycles:H -C 33 -d -d -I 1000 sleep 1
> time counts unit events
> 1.001006400 3,298,348,656 cycles:G (70.03%)
> 1.001006400 3,144,532 cycles:H (70.03%)
> 1.001006400 941,149 L1-dcache-loads (70.03%)
> 1.001006400 17,937 L1-dcache-load-misses # 1.91% of all L1-dcache accesses (70.03%)
> 1.001006400 <not supported> LLC-loads
> 1.001006400 <not supported> LLC-load-misses
> 1.001006400 1,101,889 L1-icache-loads (70.03%)
> 1.001006400 121,638 L1-icache-load-misses # 11.04% of all L1-icache accesses (70.03%)
> 1.001006400 1,031,228 dTLB-loads (70.03%)
> 1.001006400 26,952 dTLB-load-misses # 2.61% of all dTLB cache accesses (69.93%)
> 1.001006400 1,030,678 iTLB-loads (59.94%)
> 1.001006400 338 iTLB-load-misses # 0.03% of all iTLB cache accesses (59.94%)
>
> The result is correct. The "cycle:G" is nearly 3.3G now.
>
> Signed-off-by: Huang Shijie <shijie@...amperecomputing.com>
> ---
> v1 --> v2:
> Do not change perf/core code, only change the ARM64 kvm code.
> v1: https://lkml.org/lkml/2023/8/8/1465
>
> ---
> arch/arm64/kvm/arm.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index c2c14059f6a8..475a2f0e0e40 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -919,8 +919,17 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> if (!ret)
> ret = 1;
>
> - if (ret > 0)
> + if (ret > 0) {
> + /*
> + * The perf_rotate_context() may rotate the events and
> + * reprogram PMU with filters for host context.
> + * So make a request before reentering the guest to
> + * reconfigurate the event filters for guest context.
> + */
> + kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
> +
> ret = check_vcpu_requests(vcpu);
> + }
This looks extremely heavy handed. You're performing the reload on
*every* entry, and I don't think this is right (exit-heavy workloads
will suffer from it)
Furthermore, you're also reloading the virtual state of the PMU
(recreating guest events and other things), all of which looks pretty
pointless, as all we're interested in is what is being counted on the
*host*.
Instead, we can restrict the reload of the host state (and only that)
to situations where:
- we're running on a VHE system
- we have a host PMUv3 (not everybody does), as that's the only way we
can profile a guest
and ideally we would have a way to detect that a rotation happened
(which may requires some help from the low-level PMU code).
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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