Messages in this thread | | | Date | Tue, 28 Feb 2023 20:39:30 +0000 | From | Joel Fernandes <> | Subject | Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall |
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On Tue, Feb 28, 2023 at 04:51:21PM +0100, Uros Bizjak wrote: > Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in > check_cpu_stall. x86 CMPXCHG instruction returns success in ZF flag, so > this change saves a compare after cmpxchg (and related move instruction in > front of cmpxchg).
In my codegen, I am not seeing mov instruction before the cmp removed, how can that be? The rax has to be populated with a mov before cmpxchg right?
So try_cmpxchg gives: mov, cmpxchg, cmp, jne Where as cmpxchg gives: mov, cmpxchg, mov, jne
So yeah you got rid of compare, but I am not seeing reduction in moves. Either way, I think it is an improvement due to dropping cmp so:
Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
thanks,
- Joel
> > No functional change intended. > > Cc: "Paul E. McKenney" <paulmck@kernel.org> > Cc: Frederic Weisbecker <frederic@kernel.org> > Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com> > Cc: Josh Triplett <josh@joshtriplett.org> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > Cc: Joel Fernandes <joel@joelfernandes.org> > Signed-off-by: Uros Bizjak <ubizjak@gmail.com> > --- > kernel/rcu/tree_stall.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h > index b10b8349bb2a..d81c88e66b42 100644 > --- a/kernel/rcu/tree_stall.h > +++ b/kernel/rcu/tree_stall.h > @@ -760,7 +760,7 @@ static void check_cpu_stall(struct rcu_data *rdp) > jn = jiffies + ULONG_MAX / 2; > if (rcu_gp_in_progress() && > (READ_ONCE(rnp->qsmask) & rdp->grpmask) && > - cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) { > + try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) { > > /* > * If a virtual machine is stopped by the host it can look to > @@ -778,7 +778,7 @@ static void check_cpu_stall(struct rcu_data *rdp) > > } else if (rcu_gp_in_progress() && > ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) && > - cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) { > + try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) { > > /* > * If a virtual machine is stopped by the host it can look to > -- > 2.39.2 >
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