linux-kernel - Re: [PATCH v2 0/4] /proc/stat: Reduce irqs counting performance overhead

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Date:   Wed, 9 Jan 2019 15:14:38 -0500
From:   Waiman Long <longman@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.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>
Subject: Re: [PATCH v2 0/4] /proc/stat: Reduce irqs counting performance
 overhead

On 01/09/2019 02:59 PM, Matthew Wilcox wrote:
> On Wed, Jan 09, 2019 at 01:54:36PM -0500, Waiman Long wrote:
>> If you read patch 4, you can see that quite a bit of CPU cycles was
>> spent looking up the radix tree to locate the IRQ descriptor for each of
>> the interrupts. Those overhead will still be there even if I use percpu
>> counters. So using percpu counter alone won't be as performant as this
>> patch or my previous v1 patch.
> Hm, if that's the overhead, then the radix tree (and the XArray) have
> APIs that can reduce that overhead.  Right now, there's only one caller
> of kstat_irqs_usr() (the proc code).  If we change that to fill an array
> instead of returning a single value, it can look something like this:
>
> void kstat_irqs_usr(unsigned int *sums)
> {
> 	XA_STATE(xas, &irq_descs, 0);
> 	struct irq_desc *desc;
>
> 	xas_for_each(&xas, desc, ULONG_MAX) {
> 		unsigned int sum = 0;
>
> 		if (!desc->kstat_irqs)
> 			continue;
> 		for_each_possible_cpu(cpu)
> 			sum += *per_cpu_ptr(desc->kstat_irqs, cpu);
>
> 		sums[xas->xa_index] = sum;
> 	}
> }

OK, I will try something like that as a replacement of patch 4 to see
how it compares with my current patch.

Thanks,
Longman