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locking/qspinlock: Micro-optimize pending state waiting for unlock

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When we're pending, we only care about lock value. The xchg_tail
wouldn't affect the pending state. That means the hardware thread
could stay in a sleep state and leaves the rest execution units'
resources of pipeline to other hardware threads. This situation is
the SMT scenarios in the same core. Not an entering low-power state
situation. Of course, the granularity between cores is "cacheline",
but the granularity between SMT hw threads of the same core could
be "byte" which internal LSU handles. For example, when a hw-thread
yields the resources of the core to other hw-threads, this patch
could help the hw-thread stay in the sleep state and prevent it
from being woken up by other hw-threads xchg_tail.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/20230105021952.3090070-1-guoren@kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
This commit is contained in:
Guo Ren 2023-01-04 21:19:52 -05:00 committed by Ingo Molnar
parent 512dee0c00
commit 4282494a20
1 changed files with 2 additions and 2 deletions
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4
kernel/locking/qspinlock.c
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@ -371,7 +371,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
/*
* We're pending, wait for the owner to go away.
*
* 0,1,1 -> 0,1,0
* 0,1,1 -> *,1,0
*
* this wait loop must be a load-acquire such that we match the
* store-release that clears the locked bit and create lock
@ -380,7 +380,7 @@ void __lockfunc queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
* barriers.
*/
if (val & _Q_LOCKED_MASK)
atomic_cond_read_acquire(&lock->val, !(VAL & _Q_LOCKED_MASK));
smp_cond_load_acquire(&lock->locked, !VAL);
/*
* take ownership and clear the pending bit.