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ksoftirqd: Enable IRQs and call cond_resched() before poking RCU

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While debugging an issue with excessive softirq usage, I encountered the
following note in commit 3e339b5dae ("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
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Calvin Owens 2015-01-13 13:16:18 -08:00 committed by Paul E. McKenney
parent 5a43b88e98
commit 28423ad283
1 changed files with 5 additions and 1 deletions
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6
kernel/softirq.c
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@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int cpu)
* in the task stack here.
*/
__do_softirq();
rcu_note_context_switch();
local_irq_enable();
cond_resched();
preempt_disable();
rcu_note_context_switch();
preempt_enable();
return;
}
local_irq_enable();