Messages in this thread | | | From | Joel Fernandes <> | Subject | Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall | Date | Tue, 28 Feb 2023 15:56:09 -0500 |
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> On Feb 28, 2023, at 3:39 PM, Joel Fernandes <joel@joelfernandes.org> wrote: > > 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>
Ah there probably is a lesser mov to hold the value to compare against, but maybe not always?
Would be nice to clarify that in the changelog anyway.
Thanks,
- Joel
> > 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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