Re: [PATCH] random: Optimize add_interrupt_randomness

["Theodore Ts'o" <tytso@mit.edu>]

Thanks, applied.

					- Ted

On Wed, Feb 28, 2018 at 01:43:28PM -0800, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> add_interrupt_randomess always wakes up
> code blocking on /dev/random. This wake up is done
> unconditionally. Unfortunately this means all interrupts
> take the wait queue spinlock, which can be rather expensive
> on large systems processing lots of interrupts.
> 
> We saw 1% cpu time spinning on this on a large macro workload
> running on a large system.
> 
> I believe it's a recent regression (?)
> 
> Always check if there is a waiter on the wait queue
> before waking up. This check can be done without
> taking a spinlock.
> 
> 1.06%         10460  [kernel.vmlinux] [k] native_queued_spin_lock_slowpath
>          |
>          ---native_queued_spin_lock_slowpath
>             |
>              --0.57%--_raw_spin_lock_irqsave
>                        |
>                         --0.56%--__wake_up_common_lock
>                                   credit_entropy_bits
>                                   add_interrupt_randomness
>                                   handle_irq_event_percpu
>                                   handle_irq_event
>                                   handle_edge_irq
>                                   handle_irq
>                                   do_IRQ
>                                   common_interrupt
> 
> Signed-off-by: Andi Kleen <ak@linux.intel.com>