linux-kernel - [PATCH 2/6] x86/pvclock: improve atomic update of last_value in pvclock_clocksource

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Date:   Mon, 23 Jan 2023 21:50:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org
Cc:     will@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
        mark.rutland@....com, tglx@...utronix.de, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        seanjc@...gle.com, pbonzini@...hat.com, jgross@...e.com,
        srivatsa@...il.mit.edu, amakhalov@...are.com,
        pv-drivers@...are.com, rostedt@...dmis.org, mhiramat@...nel.org,
        wanpengli@...cent.com, vkuznets@...hat.com,
        boris.ostrovsky@...cle.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        linux-trace-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Uros Bizjak <ubizjak@...il.com>
Subject: [PATCH 2/6] x86/pvclock: improve atomic update of last_value in pvclock_clocksource_read

From: Uros Bizjak <ubizjak@...il.com>

Improve atomic update of last_value in pvclock_clocksource_read:

- Atomic update can be skipped if the "last_value" is already
  equal to "ret".

- The detection of atomic update failure is not correct. The value,
  returned by atomic64_cmpxchg should be compared to the old value
  from the location to be updated. If these two are the same, then
  atomic update succeeded and "last_value" location is updated to
  "ret" in an atomic way. Otherwise, the atomic update failed and
  it should be retried with the value from "last_value" - exactly
  what atomic64_try_cmpxchg does in a correct and more optimal way.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20230118202330.3740-1-ubizjak@gmail.com
---
 arch/x86/kernel/pvclock.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index eda37df016f0..5a2a517dd61b 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -102,10 +102,9 @@ u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 	 */
 	last = atomic64_read(&last_value);
 	do {
-		if (ret < last)
+		if (ret <= last)
 			return last;
-		last = atomic64_cmpxchg(&last_value, last, ret);
-	} while (unlikely(last != ret));
+	} while (!atomic64_try_cmpxchg(&last_value, &last, ret));
 
 	return ret;
 }
-- 
2.39.0