futex: Optimize per-cpu reference counting
Shrikanth noted that the per-cpu reference counter was still some 10%
slower than the old immutable option (which removes the reference
counting entirely).
Further optimize the per-cpu reference counter by:
- switching from RCU to preempt;
- using __this_cpu_*() since we now have preempt disabled;
- switching from smp_load_acquire() to READ_ONCE().
This is all safe because disabling preemption inhibits the RCU grace
period exactly like rcu_read_lock().
Having preemption disabled allows using __this_cpu_*() provided the
only access to the variable is in task context -- which is the case
here.
Furthermore, since we know changing fph->state to FR_ATOMIC demands a
full RCU grace period we can rely on the implied smp_mb() from that to
replace the acquire barrier().
This is very similar to the percpu_down_read_internal() fast-path.
The reason this is significant for PowerPC is that it uses the generic
this_cpu_*() implementation which relies on local_irq_disable() (the
x86 implementation relies on it being a single memop instruction to be
IRQ-safe). Switching to preempt_disable() and __this_cpu*() avoids
this IRQ state swizzling. Also, PowerPC needs LWSYNC for the ACQUIRE
barrier, not having to use explicit barriers safes a bunch.
Combined this reduces the performance gap by half, down to some 5%.
Fixes: 760e6f7befba ("futex: Remove support for IMMUTABLE")
Reported-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20251106092929.GR4067720@noisy.programming.kicks-ass.net
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 125804f..2e77a6e 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1680,10 +1680,10 @@ static bool futex_ref_get(struct futex_private_hash *fph)
{
struct mm_struct *mm = fph->mm;
- guard(rcu)();
+ guard(preempt)();
- if (smp_load_acquire(&fph->state) == FR_PERCPU) {
- this_cpu_inc(*mm->futex_ref);
+ if (READ_ONCE(fph->state) == FR_PERCPU) {
+ __this_cpu_inc(*mm->futex_ref);
return true;
}
@@ -1694,10 +1694,10 @@ static bool futex_ref_put(struct futex_private_hash *fph)
{
struct mm_struct *mm = fph->mm;
- guard(rcu)();
+ guard(preempt)();
- if (smp_load_acquire(&fph->state) == FR_PERCPU) {
- this_cpu_dec(*mm->futex_ref);
+ if (READ_ONCE(fph->state) == FR_PERCPU) {
+ __this_cpu_dec(*mm->futex_ref);
return false;
}
