From: Leo Yan <leo.yan@linaro.org>
To: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
Mathieu Poirier <mathieu.poirier@linaro.org>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Mike Leach <mike.leach@linaro.org>,
Andi Kleen <ak@linux.intel.com>,
linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 8/8] perf record: Directly bail out for compat case
Date: Wed, 2 Jun 2021 20:38:47 +0800 [thread overview]
Message-ID: <20210602123847.GE10272@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <c321e998-6fd2-86e9-7876-7250a9b23c25@intel.com>
Hi Adrain,
On Wed, Jun 02, 2021 at 02:18:47PM +0300, Adrian Hunter wrote:
> On 2/06/21 1:30 pm, Leo Yan wrote:
> > Since the 64-bit atomicity is not promised in 32-bit perf, directly
> > report the error and bail out for this case.
> >
> > Now only applies on x86_64 and Arm64 platforms.
> >
> > Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
>
> Maybe we can do better for the compat case.
>
> We can assume the upper 32-bits change very seldom,
> and always increase. So for the 'read' case:
>
> u64 first, second, last;
> u64 mask = (u64)((u32)-1) << 32;
>
> do {
> first = READ_ONCE(pc->aux_head);
> rmb();
> second = READ_ONCE(pc->aux_head);
> rmb();
> last = READ_ONCE(pc->aux_head);
> } while ((first & mask) != (last & mask));
> return second;
>
> For the write case, we can cause a fatal error only if the new
> tail has non-zero upper 32-bits. That gives up to 4GiB of data
> before aborting:
>
> if (tail & mask)
> return -1;
> smp_mb();
> WRITE_ONCE(pc->aux_tail, tail);
Seems to me, it's pointless to only support aux_head for 64-bit and
support aux_tail for 32-bit. I understand this can be helpful for the
snapshot mode which only uses aux_head, but it still fails to support
the normal case for AUX ring buffer using 64-bit head/tail.
Thanks,
Leo
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next prev parent reply other threads:[~2021-06-02 13:04 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-02 10:29 [PATCH v2 0/8] perf: Refine barriers for AUX ring buffer Leo Yan
2021-06-02 10:30 ` [PATCH v2 1/8] perf/ring_buffer: Add comment for barriers on " Leo Yan
2021-06-07 15:27 ` Peter Zijlstra
2021-06-02 10:30 ` [PATCH v2 2/8] coresight: tmc-etr: Add barrier after updating " Leo Yan
2021-06-02 10:30 ` [PATCH v2 3/8] coresight: tmc-etf: Add comment for store ordering Leo Yan
2021-06-02 10:30 ` [PATCH v2 4/8] perf/x86: Add barrier after updating bts Leo Yan
2021-06-07 15:29 ` Peter Zijlstra
2021-06-02 10:30 ` [PATCH v2 5/8] perf auxtrace: Change to use SMP memory barriers Leo Yan
2021-06-07 10:02 ` Adrian Hunter
2021-06-07 15:29 ` Peter Zijlstra
2021-06-08 16:45 ` Arnaldo Carvalho de Melo
2021-06-02 10:30 ` [PATCH v2 6/8] perf auxtrace: Drop legacy __sync functions Leo Yan
2021-06-02 10:47 ` Adrian Hunter
2021-06-02 11:16 ` Leo Yan
2021-06-02 11:21 ` Adrian Hunter
2021-06-02 13:01 ` Leo Yan
2021-06-02 10:30 ` [PATCH v2 7/8] perf auxtrace: Use WRITE_ONCE() for updating aux_tail Leo Yan
2021-06-07 10:03 ` Adrian Hunter
2021-06-07 15:31 ` Peter Zijlstra
2021-06-08 17:04 ` Arnaldo Carvalho de Melo
2021-06-09 0:21 ` Leo Yan
2021-06-02 10:30 ` [PATCH v2 8/8] perf record: Directly bail out for compat case Leo Yan
2021-06-02 11:18 ` Adrian Hunter
2021-06-02 12:38 ` Leo Yan [this message]
2021-06-07 10:23 ` Adrian Hunter
2021-06-07 15:09 ` Leo Yan
2021-06-09 8:23 ` Adrian Hunter
2021-06-09 8:57 ` Leo Yan
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