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From: Leo Yan <leo.yan@linaro.org>
To: Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Paul Cercueil <paul@crapouillou.net>,
	"Ben Dooks (Codethink)" <ben.dooks@codethink.co.uk>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Kan Liang <kan.liang@linux.intel.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: [PATCH v2 2/6] arm64: perf: Implement correct cap_user_time
Date: Wed, 15 Jul 2020 10:05:08 +0800	[thread overview]
Message-ID: <20200715020512.20991-3-leo.yan@linaro.org> (raw)
In-Reply-To: <20200715020512.20991-1-leo.yan@linaro.org>

From: Peter Zijlstra <peterz@infradead.org>

As reported by Leo; the existing implementation is broken when the
clock and counter don't intersect at 0.

Use the sched_clock's struct clock_read_data information to correctly
implement cap_user_time and cap_user_time_zero.

Note that the ARM64 counter is architecturally only guaranteed to be
56bit wide (implementations are allowed to be wider) and the existing
perf ABI cannot deal with wrap-around.

This implementation should also be faster than the old; seeing how we
don't need to recompute mult and shift all the time.

[leoyan: Use quot/rem to convert cyc to ns to avoid overflow]

Reported-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/arm64/kernel/perf_event.c | 40 ++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index 4d7879484cec..35c2c737d4af 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -19,6 +19,7 @@
 #include <linux/of.h>
 #include <linux/perf/arm_pmu.h>
 #include <linux/platform_device.h>
+#include <linux/sched_clock.h>
 #include <linux/smp.h>
 
 /* ARMv8 Cortex-A53 specific event types. */
@@ -1165,28 +1166,49 @@ device_initcall(armv8_pmu_driver_init)
 void arch_perf_update_userpage(struct perf_event *event,
 			       struct perf_event_mmap_page *userpg, u64 now)
 {
-	u32 freq;
-	u32 shift;
+	struct clock_read_data *rd;
+	unsigned int seq;
+	u64 quot, rem, ns;
 
 	/*
 	 * Internal timekeeping for enabled/running/stopped times
 	 * is always computed with the sched_clock.
 	 */
-	freq = arch_timer_get_rate();
 	userpg->cap_user_time = 1;
+	userpg->cap_user_time_zero = 1;
+
+	do {
+		rd = sched_clock_read_begin(&seq);
+
+		userpg->time_mult = rd->mult;
+		userpg->time_shift = rd->shift;
+		userpg->time_zero = rd->epoch_ns;
+
+		/*
+		 * This isn't strictly correct, the ARM64 counter can be
+		 * 'short' and then we get funnies when it wraps. The correct
+		 * thing would be to extend the perf ABI with a cycle and mask
+		 * value, but because wrapping on ARM64 is very rare in
+		 * practise this 'works'.
+		 */
+		quot = rd->epoch_cyc >> rd->shift;
+		rem = rd->epoch_cyc & (((u64)1 << rd->shift) - 1);
+		ns = quot * rd->mult + ((rem * rd->mult) >> rd->shift);
+		userpg->time_zero -= ns;
+
+	} while (sched_clock_read_retry(seq));
+
+	userpg->time_offset = userpg->time_zero - now;
 
-	clocks_calc_mult_shift(&userpg->time_mult, &shift, freq,
-			NSEC_PER_SEC, 0);
 	/*
 	 * time_shift is not expected to be greater than 31 due to
 	 * the original published conversion algorithm shifting a
 	 * 32-bit value (now specifies a 64-bit value) - refer
 	 * perf_event_mmap_page documentation in perf_event.h.
 	 */
-	if (shift == 32) {
-		shift = 31;
+	if (userpg->time_shift == 32) {
+		userpg->time_shift = 31;
 		userpg->time_mult >>= 1;
 	}
-	userpg->time_shift = (u16)shift;
-	userpg->time_offset = -now;
+
 }
-- 
2.17.1


  parent reply	other threads:[~2020-07-15  2:05 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-15  2:05 [PATCH v2 0/6] arm64: perf: Proper cap_user_time* support Leo Yan
2020-07-15  2:05 ` [PATCH v2 1/6] sched_clock: Expose struct clock_read_data Leo Yan
2020-07-15  5:56   ` Ahmed S. Darwish
2020-07-15  6:54     ` Leo Yan
2020-07-15  7:21       ` Ahmed S. Darwish
2020-07-15  8:12     ` Peter Zijlstra
2020-07-15  8:14       ` peterz
2020-07-15  9:23         ` Ahmed S. Darwish
2020-07-15  9:52           ` Peter Zijlstra
2020-07-15 11:59             ` [PATCH] time/sched_clock: Use raw_read_seqcount_latch() Ahmed S. Darwish
2020-07-15 15:29               ` Leo Yan
2020-07-15 15:58                 ` Peter Zijlstra
2020-07-16  5:22                   ` Leo Yan
2020-09-10 15:08           ` [tip: locking/core] time/sched_clock: Use raw_read_seqcount_latch() during suspend tip-bot2 for Ahmed S. Darwish
2020-07-15  2:05 ` Leo Yan [this message]
2020-07-15  8:38   ` [PATCH v2 2/6] arm64: perf: Implement correct cap_user_time Peter Zijlstra
2020-07-15 15:39     ` Leo Yan
2020-07-15  2:05 ` [PATCH v2 3/6] arm64: perf: Only advertise cap_user_time for arch_timer Leo Yan
2020-07-15  2:05 ` [PATCH v2 4/6] perf: Add perf_event_mmap_page::cap_user_time_short ABI Leo Yan
2020-07-15  2:05 ` [PATCH v2 5/6] arm64: perf: Add cap_user_time_short Leo Yan
2020-07-15  2:05 ` [PATCH v2 6/6] tools headers UAPI: Update tools's copy of linux/perf_event.h Leo Yan
2020-07-15  5:17 ` [PATCH v2 0/6] arm64: perf: Proper cap_user_time* support Ahmed S. Darwish
2020-07-15  6:29   ` Leo Yan

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