Re: [PATCH] sched: Add missing memory barrier in switch_mm_cid
From: Mathieu Desnoyers
Date: Wed Apr 10 2024 - 11:29:43 EST
Peter, Ingo, can you please consider merging this fix ?
It adds a missing memory barrier in switch_mm_cid(), which is a
no-op on x86 because switch_mm already provides it. This issue was
reported more than a month ago.
Thanks,
Mathieu
On 2024-04-08 05:38, Yeo Reum Yun wrote:
Gentle ping...
________________________________________
From: Yeo Reum Yun <YeoReum.Yun@xxxxxxx>
Sent: 19 March 2024 09:20
To: Mathieu Desnoyers; Ingo Molnar; Peter Zijlstra
Cc: linux-kernel@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx; Steven Rostedt; Vincent Guittot; Juri Lelli; Dietmar Eggemann; Ben Segall; Mel Gorman; Daniel Bristot de Oliveira; Valentin Schneider; Catalin Marinas; Mark Rutland; Will Deacon; Aaron Lu
Subject: Re: [PATCH] sched: Add missing memory barrier in switch_mm_cid
Gentle ping.
________________________________________
From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Sent: 08 March 2024 15:07
To: Ingo Molnar; Peter Zijlstra
Cc: linux-kernel@xxxxxxxxxxxxxxx; Mathieu Desnoyers; Yeo Reum Yun; stable@xxxxxxxxxxxxxxx; Steven Rostedt; Vincent Guittot; Juri Lelli; Dietmar Eggemann; Ben Segall; Mel Gorman; Daniel Bristot de Oliveira; Valentin Schneider; Catalin Marinas; Mark Rutland; Will Deacon; Aaron Lu
Subject: [PATCH] sched: Add missing memory barrier in switch_mm_cid
Many architectures' switch_mm() (e.g. arm64) do not have an smp_mb()
which the core scheduler code has depended upon since commit:
commit 223baf9d17f25 ("sched: Fix performance regression introduced by mm_cid")
If switch_mm() doesn't call smp_mb(), sched_mm_cid_remote_clear() can
unset the actively used cid when it fails to observe active task after it
sets lazy_put.
There *is* a memory barrier between storing to rq->curr and _return to
userspace_ (as required by membarrier), but the rseq mm_cid has stricter
requirements: the barrier needs to be issued between store to rq->curr
and switch_mm_cid(), which happens earlier than:
- spin_unlock(),
- switch_to().
So it's fine when the architecture switch_mm happens to have that barrier
already, but less so when the architecture only provides the full barrier
in switch_to() or spin_unlock().
It is a bug in the rseq switch_mm_cid() implementation. All architectures
that don't have memory barriers in switch_mm(), but rather have the full
barrier either in finish_lock_switch() or switch_to() have them too late
for the needs of switch_mm_cid().
Introduce a new smp_mb__after_switch_mm(), defined as smp_mb() in the
generic barrier.h header, and use it in switch_mm_cid() for scheduler
transitions where switch_mm() is expected to provide a memory barrier.
Architectures can override smp_mb__after_switch_mm() if their
switch_mm() implementation provides an implicit memory barrier.
Override it with a no-op on x86 which implicitly provide this memory
barrier by writing to CR3.
Link: https://lore.kernel.org/lkml/20240305145335.2696125-1-yeoreum.yun@xxxxxxx/
Reported-by: levi.yun <yeoreum.yun@xxxxxxx>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by mm_cid")
Cc: <stable@xxxxxxxxxxxxxxx> # 6.4.x
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: levi.yun <yeoreum.yun@xxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
---
arch/x86/include/asm/barrier.h | 3 +++
include/asm-generic/barrier.h | 8 ++++++++
kernel/sched/sched.h | 20 ++++++++++++++------
3 files changed, 25 insertions(+), 6 deletions(-)
diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 35389b2af88e..0d5e54201eb2 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -79,6 +79,9 @@ do { \
#define __smp_mb__before_atomic() do { } while (0)
#define __smp_mb__after_atomic() do { } while (0)
+/* Writing to CR3 provides a full memory barrier in switch_mm(). */
+#define smp_mb__after_switch_mm() do { } while (0)
+
#include <asm-generic/barrier.h>
/*
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 961f4d88f9ef..5a6c94d7a598 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -296,5 +296,13 @@ do { \
#define io_stop_wc() do { } while (0)
#endif
+/*
+ * Architectures that guarantee an implicit smp_mb() in switch_mm()
+ * can override smp_mb__after_switch_mm.
+ */
+#ifndef smp_mb__after_switch_mm
+#define smp_mb__after_switch_mm() smp_mb()
+#endif
+
#endif /* !__ASSEMBLY__ */
#endif /* __ASM_GENERIC_BARRIER_H */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 2e5a95486a42..044d842c696c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -79,6 +79,8 @@
# include <asm/paravirt_api_clock.h>
#endif
+#include <asm/barrier.h>
+
#include "cpupri.h"
#include "cpudeadline.h"
@@ -3481,13 +3483,19 @@ static inline void switch_mm_cid(struct rq *rq,
* between rq->curr store and load of {prev,next}->mm->pcpu_cid[cpu].
* Provide it here.
*/
- if (!prev->mm) // from kernel
+ if (!prev->mm) { // from kernel
smp_mb();
- /*
- * user -> user transition guarantees a memory barrier through
- * switch_mm() when current->mm changes. If current->mm is
- * unchanged, no barrier is needed.
- */
+ } else { // from user
+ /*
+ * user -> user transition relies on an implicit
+ * memory barrier in switch_mm() when
+ * current->mm changes. If the architecture
+ * switch_mm() does not have an implicit memory
+ * barrier, it is emitted here. If current->mm
+ * is unchanged, no barrier is needed.
+ */
+ smp_mb__after_switch_mm();
+ }
}
if (prev->mm_cid_active) {
mm_cid_snapshot_time(rq, prev->mm);
--
2.39.2
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--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com