Messages in this thread | | | Date | Tue, 20 Jan 2015 12:30:22 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU |
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On Tue, Jan 20, 2015 at 02:21:51PM +0100, Thomas Gleixner wrote: > On Tue, 13 Jan 2015, Calvin Owens wrote: > > > While debugging an issue with excessive softirq usage, I encountered the > > following note in commit 3e339b5dae24a706 ("softirq: Use hotplug thread > > infrastructure"): > > > > [ paulmck: Call rcu_note_context_switch() with interrupts enabled. ] > > > > ...but despite this note, the patch still calls RCU with IRQs disabled. > > > > This seemingly innocuous change caused a significant regression in softirq > > CPU usage on the sending side of a large TCP transfer (~1 GB/s): when > > introducing 0.01% packet loss, the softirq usage would jump to around 25%, > > spiking as high as 50%. Before the change, the usage would never exceed 5%. > > > > Moving the call to rcu_note_context_switch() after the cond_sched() call, > > as it was originally before the hotplug patch, completely eliminated this > > problem. > > > > Signed-off-by: Calvin Owens <calvinowens@fb.com> > > Cc: stable@vger.kernel.org > > --- > > This version includes the "cpu" argument to rcu_note_context_switch() in > > order to apply cleanly to stable kernels. It will need to be removed to > > apply to 3.18+ and 3.19 (upstream commit 38200cf2 removed the argument). > > > > kernel/softirq.c | 6 +++++- > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/softirq.c b/kernel/softirq.c > > index 501baa9..9e787d8 100644 > > --- a/kernel/softirq.c > > +++ b/kernel/softirq.c > > @@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int cpu) > > * in the task stack here. > > */ > > __do_softirq(); > > - rcu_note_context_switch(cpu); > > local_irq_enable(); > > cond_resched(); > > + > > + preempt_disable(); > > + rcu_note_context_switch(cpu); > > + preempt_enable(); > > + > > The whole rcu_note_context_switch() in run_ksoftirqd() is silly. > > cond_resched() > __preempt_count_add(PREEMPT_ACTIVE); > > __schedule(); > preempt_disable(); > rcu_note_context_switch(); > .... > > __preempt_count_sub(PREEMPT_ACTIVE);
I agree that if should_resched() returns true as assumed above, then there is no point to invoking rcu_note_context_switch(). However, the case that this code applies to is when should_resched() returns false, but RCU is waiting for a quiescent state from the current CPU. In that case, cond_resched() won't do anything for RCU, and we do need the rcu_note_context_switch().
Of course, it would be better to avoid the extra RCU work in the common case where cond_resched() does inovke the scheduler. And that is the point of the following patch, which uses cond_resched_rcu_qs(). However, this use of cond_resched_rcu_qs() doesn't work in older kernels. So Calvin's patch is for backporting, and the follow-up patch for future kernels.
Make sense, or am I missing something?
Thanx, Paul
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