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Date: Thu, 19 Apr 2018 23:02:17 +0300
From: Alexey Dobriyan <adobriyan@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Waiman Long <longman@...hat.com>,
Kate Stewart <kstewart@...uxfoundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] proc/stat: Separate out individual irq counts into
/proc/stat_irqs
On Thu, Apr 19, 2018 at 12:43:19PM -0700, Andrew Morton wrote:
> On Thu, 19 Apr 2018 13:09:29 -0400 Waiman Long <longman@...hat.com> wrote:
>
> > It was found that reading /proc/stat could be time consuming on
> > systems with a lot of irqs. For example, reading /proc/stat in a
> > certain 2-socket Skylake server took about 4.6ms because it had over
> > 5k irqs. In that particular case, the majority of the CPU cycles for
> > reading /proc/stat was spent in the kstat_irqs() function. Therefore,
> > application performance can be impacted if the application reads
> > /proc/stat rather frequently.
> >
> > The "intr" line within /proc/stat contains a sum total of all the irqs
> > that have happened followed by a list of irq counts for each individual
> > irq number. In many cases, the first number is good enough. The
> > individual irq counts may not provide that much more information.
> >
> > In order to avoid this kind of performance issue, all these individual
> > irq counts are now separated into a new /proc/stat_irqs file. The
> > sum total irq count will stay in /proc/stat and be duplicated in
> > /proc/stat_irqs. Applications that need to look up individual irq counts
> > will now have to look into /proc/stat_irqs instead of /proc/stat.
> >
>
> (cc /proc maintainer)
>
> It's a non-backward-compatible change. For something which has
> existing for so long, it would be a mighty task to demonstrate that no
> existing userspace will be disrupted by this change.
>
> So we need to think again. A new interface which omits the per-IRQ
> stats might be acceptable.
Here is profile of open+read+close /proc/stat
30% is taking mutex only to print "0".
+ 98.80% 0.04% a.out [kernel.vmlinux] [k] entry_SYSCALL_64 ▒
+ 98.75% 0.10% a.out [kernel.vmlinux] [k] do_syscall_64 ▒
+ 95.56% 0.04% a.out libc-2.25.so [.] __GI___libc_read ◆
+ 95.09% 0.01% a.out [kernel.vmlinux] [k] sys_read ▒
+ 95.04% 0.03% a.out [kernel.vmlinux] [k] vfs_read ▒
+ 94.98% 0.05% a.out [kernel.vmlinux] [k] proc_reg_read ▒
+ 94.98% 0.00% a.out [kernel.vmlinux] [k] __vfs_read ▒
+ 94.92% 0.06% a.out [kernel.vmlinux] [k] seq_read ▒
+ 94.52% 3.65% a.out [kernel.vmlinux] [k] show_stat ▒
+ 48.62% 2.59% a.out [kernel.vmlinux] [k] kstat_irqs_usr ▒
+ 33.52% 9.55% a.out [kernel.vmlinux] [k] seq_put_decimal_ull ▒
+ 19.63% 19.59% a.out [kernel.vmlinux] [k] memcpy_erms ▒
+ 17.34% 9.53% a.out [kernel.vmlinux] [k] kstat_irqs ▒
- 15.45% 15.43% a.out [kernel.vmlinux] [k] mutex_lock ▒
15.43% __GI___libc_read ▒
entry_SYSCALL_64 ▒
do_syscall_64 ▒
sys_read ▒
vfs_read ▒
__vfs_read ▒
proc_reg_read ▒
- seq_read ▒
- 15.41% show_stat ▒
kstat_irqs_usr ▒
mutex_lock ▒
+ 13.32% 13.27% a.out [kernel.vmlinux] [k] mutex_unlock ▒
+ 4.60% 1.35% a.out [kernel.vmlinux] [k] cpumask_next ▒
+ 3.03% 3.03% a.out [kernel.vmlinux] [k] __radix_tree_lookup ▒
+ 2.96% 0.08% a.out [kernel.vmlinux] [k] seq_printf ▒
+ 2.92% 0.02% a.out libc-2.25.so [.] __GI___libc_open ▒
+ 2.89% 0.07% a.out [kernel.vmlinux] [k] seq_vprintf ▒
+ 2.81% 0.70% a.out [kernel.vmlinux] [k] vsnprintf ▒
+ 2.66% 2.66% a.out [kernel.vmlinux] [k] _find_next_bit ▒
+ 2.42% 1.36% a.out [kernel.vmlinux] [k] num_to_str ▒
+ 2.41% 0.19% a.out [kernel.vmlinux] [k] get_idle_time ▒
+ 2.39% 0.02% a.out [kernel.vmlinux] [k] do_sys_open
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