linux-kernel - [PATCH] drm/i915/pmu: Use local64_try_cmpxchg in i915_pmu_event

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Message-ID: <20230703150859.6176-1-ubizjak@gmail.com>
Date:   Mon,  3 Jul 2023 17:08:18 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: [PATCH] drm/i915/pmu: Use local64_try_cmpxchg in i915_pmu_event_read

Use local64_try_cmpxchg instead of local64_cmpxchg (*ptr, old, new) == old
in i915_pmu_event_read.  x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

No functional change intended.

Cc: Jani Nikula <jani.nikula@...ux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Cc: David Airlie <airlied@...il.com>
Cc: Daniel Vetter <daniel@...ll.ch>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 drivers/gpu/drm/i915/i915_pmu.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index d35973b41186..108b675088ba 100644
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -696,12 +696,11 @@ static void i915_pmu_event_read(struct perf_event *event)
 		event->hw.state = PERF_HES_STOPPED;
 		return;
 	}
-again:
-	prev = local64_read(&hwc->prev_count);
-	new = __i915_pmu_event_read(event);
 
-	if (local64_cmpxchg(&hwc->prev_count, prev, new) != prev)
-		goto again;
+	prev = local64_read(&hwc->prev_count);
+	do {
+		new = __i915_pmu_event_read(event);
+	} while (!local64_try_cmpxchg(&hwc->prev_count, &prev, new));
 
 	local64_add(new - prev, &event->count);
 }
-- 
2.41.0