• Calvin Owens's avatar
    ksoftirqd: Enable IRQs and call cond_resched() before poking RCU · 28423ad2
    Calvin Owens authored
    While debugging an issue with excessive softirq usage, I encountered the
    following note in commit 3e339b5d ("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: default avatarCalvin Owens <calvinowens@fb.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    28423ad2
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