Messages in this thread | | | From | Pranith Kumar <> | Date | Fri, 25 Jul 2014 16:19:43 -0400 | Subject | Re: [RFC PATCH 1/1] rcu: Use rcu_gp_kthread_wake() to wake up kthreads |
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On Fri, Jul 25, 2014 at 10:44 AM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > On Fri, Jul 25, 2014 at 01:06:58AM -0400, Pranith Kumar wrote: >> The rcu_gp_kthread_wake() function checks for three conditions before waking up >> grace period kthreads: >> >> * Is the thread we are trying to wake up the current thread? >> * Are the gp_flags zero? (all threads wait on non-zero gp_flags condition) >> * Is there no thread created for this flavour, hence nothing to wake up? >> >> If any one of these condition is true, we do not call wake_up(). >> >> In rcu_report_qs_rsp(), I added a pr_info() call testing if any of the above >> conditions is true, in which case we can avoid calling wake_up(). It turns out >> that quite a few actually are. Most of the cases where we can avoid is condition 2 >> above and condition 1 also occurs quite often. Condition 3 never happens. >> >> I could not test the wake_up() in force_quiescent_state() as that is not >> triggered trivially, but I am assuming we can replace wake_up() there too. >> >> Hence this commit tries to avoid calling wake_up() whenever we can by using >> rcu_gp_kthread_wake() function. > > This one does sound much more plausible than the earlier one. I have > a few more questions that I will ask in your follow-up message. > >> One concern is the comment which states that we need a memory barrier at that >> location which is being implied by the wake_up(). Should we put an smp_mb() and >> just not rely on the barrier provided by wake_up()? Thoughts? > > Let's see... The memory barriers are unnecessary for your case 1 > and case 3. That leaves your case 2, which is all about ->gp_flags. > It is quite possible that this case is now fully covered by locking, > so that the comment is obsolete. But why don't you check? >
I checked all the locations where gp_flags is being updated and the root node lock is held in all the cases. So I guess we can remove the comment too.
>> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com> >> --- >> kernel/rcu/tree.c | 6 ++++-- >> 1 file changed, 4 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c >> index 72e0b1f..d0e0d6e 100644 >> --- a/kernel/rcu/tree.c >> +++ b/kernel/rcu/tree.c >> @@ -1938,7 +1938,8 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags) >> { >> WARN_ON_ONCE(!rcu_gp_in_progress(rsp)); >> raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags); >> - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */ >> + /* Memory barrier implied by wake_up() path. */ >> + rcu_gp_kthread_wake(rsp); >> } >> >> /* >> @@ -2516,7 +2517,8 @@ static void force_quiescent_state(struct rcu_state *rsp) >> ACCESS_ONCE(rsp->gp_flags) = >> ACCESS_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS; >> raw_spin_unlock_irqrestore(&rnp_old->lock, flags); >> - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */ >> + /* Memory barrier implied by wake_up() path. */ >> + rcu_gp_kthread_wake(rsp); >> } >> >> /* >> -- >> 2.0.1 >> >
-- Pranith
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