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Message-ID: <ZwQzk7mH-QW13PtC@google.com>
Date: Mon, 7 Oct 2024 12:16:35 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Ravi Bangoria <ravi.bangoria@....com>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
eranian@...gle.com, mark.rutland@....com,
alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
irogers@...gle.com, adrian.hunter@...el.com,
kan.liang@...ux.intel.com, tglx@...utronix.de, bp@...en8.de,
dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
santosh.shukla@....com, ananth.narayan@....com,
sandipan.das@....com
Subject: Re: [PATCH 2/8] perf/amd/ibs: Remove pointless sample period check
Hello,
On Mon, Oct 07, 2024 at 03:48:04AM +0000, Ravi Bangoria wrote:
> Valid perf event sample period value for IBS PMUs (Fetch and Op both)
> is limited to multiple of 0x10. perf_ibs_init() has this check:
>
> if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
> return -EINVAL;
>
> But it's broken since hwc->sample_period will always be 0 when
> event->attr.sample_freq is 0 (irrespective of event->attr.freq value.)
Right, hwc->sample_period is set to event->attr.sample_period and it's
the same as sample_freq since there are in a union.
struct perf_event_attr {
__u32 type;
__u32 size;
__u64 config;
union {
__u64 sample_period;
__u64 sample_freq;
};
...
perf_event_alloc() sets the hwc->sample_period and changes it only when
both attr->freq and attr->sample_freq are not zero.
>
> One option to fix this is to change the condition:
>
> - if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
> + if (!event->attr.freq && hwc->sample_period & 0x0f)
Right, I believe this is the intention.
>
> However, that will break all userspace tools which have been using IBS
> event with sample_period not multiple of 0x10.
Correct.
>
> Another option is to remove the condition altogether and mask lower
> nibble _silently_, same as what current code is inadvertently doing.
> I'm preferring this approach as it keeps the existing behavior.
Agreed. The condition never worked and should be safe to remove.
Thanks,
Namhyung
>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@....com>
> ---
> arch/x86/events/amd/ibs.c | 9 ++-------
> 1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index 347353b9eb70..6b55a8520166 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -294,13 +294,8 @@ static int perf_ibs_init(struct perf_event *event)
> if (config & perf_ibs->cnt_mask)
> /* raw max_cnt may not be set */
> return -EINVAL;
> - if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
> - /*
> - * lower 4 bits can not be set in ibs max cnt,
> - * but allowing it in case we adjust the
> - * sample period to set a frequency.
> - */
> - return -EINVAL;
> +
> + /* Silently mask off lower nibble. IBS hw mandates it. */
> hwc->sample_period &= ~0x0FULL;
> if (!hwc->sample_period)
> hwc->sample_period = 0x10;
> --
> 2.46.2
>
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