lkml.org 
[lkml]   [2023]   [Feb]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRe: [PATCH] rcu: use try_cmpxchg in check_cpu_stall
    Date


    > 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
    >>

    \
     
     \ /
      Last update: 2023-03-27 00:38    [W:3.312 / U:0.020 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site