linux-kernel - Re: [patch 1/2] genriq: Avoid summation loops for /proc/stat

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Date:   Wed, 30 Jan 2019 11:00:09 -0500
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Miklos Szeredi <miklos@...redi.hu>,
        Daniel Colascione <dancol@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Marc Zyngier <marc.zyngier@....com>
Subject: Re: [patch 1/2] genriq: Avoid summation loops for /proc/stat

On 01/30/2019 07:31 AM, Thomas Gleixner wrote:
> Waiman reported that on large systems with a large amount of interrupts the
> readout of /proc/stat takes a long time to sum up the interrupt
> statistics. In principle this is not a problem. but for unknown reasons
> some enterprise quality software reads /proc/stat with a high frequency.
>
> The reason for this is that interrupt statistics are accounted per cpu. So
> the /proc/stat logic has to sum up the interrupt stats for each interrupt.
>
> This can be largely avoided for interrupts which are not marked as
> 'PER_CPU' interrupts by simply adding a per interrupt summation counter
> which is incremented along with the per interrupt per cpu counter.
>
> The PER_CPU interrupts need to avoid that and use only per cpu accounting
> because they share the interrupt number and the interrupt descriptor and
> concurrent updates would conflict or require unwanted synchronization.
>
> Reported-by: Waiman Long <longman@...hat.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>
> 8<-------------
>
>  include/linux/irqdesc.h |    3 ++-
>  kernel/irq/chip.c       |   12 ++++++++++--
>  kernel/irq/internals.h  |    8 +++++++-
>  kernel/irq/irqdesc.c    |    7 ++++++-
>  4 files changed, 25 insertions(+), 5 deletions(-)
>
>
> --- a/include/linux/irqdesc.h
> +++ b/include/linux/irqdesc.h
> @@ -65,9 +65,10 @@ struct irq_desc {
>  	unsigned int		core_internal_state__do_not_mess_with_it;
>  	unsigned int		depth;		/* nested irq disables */
>  	unsigned int		wake_depth;	/* nested wake enables */
> +	unsigned int		tot_count;
>  	unsigned int		irq_count;	/* For detecting broken IRQs */
> -	unsigned long		last_unhandled;	/* Aging timer for unhandled count */
>  	unsigned int		irqs_unhandled;
> +	unsigned long		last_unhandled;	/* Aging timer for unhandled count */
>  	atomic_t		threads_handled;
>  	int			threads_handled_last;
>  	raw_spinlock_t		lock;

Just one minor nit. Why you want to move the last_unhandled down one
slot? There were 5 int's before. Adding one more will just fill the
padding hole. Moving down the last_unhandled will probably leave 4-byte
holes in both above and below it assuming that raw_spinlock_t is 4 bytes.

Cheers,
Longman