Messages in this thread | | | Subject | Re: [PATCH] ARM: smp: Remove CPU: shutdown notice | From | Florian Fainelli <> | Date | Wed, 18 Jan 2017 11:56:46 -0800 |
| |
On 01/18/2017 01:55 AM, Petr Mladek wrote: > On Tue 2017-01-17 15:39:45, Florian Fainelli wrote: >> On 01/17/2017 03:23 PM, Russell King - ARM Linux wrote: >>> On Tue, Jan 17, 2017 at 03:07:12PM -0800, Florian Fainelli wrote: >>>> This message is not particularly informative, and is not paired with an >>>> identical message when a CPU is brought online. Finally, it slows the >>>> CPU hotplug path down, thus allowing less CPU hotplug operations per >>>> second. Just remove it. >>> >>> CPU hotplug isn't a fast operation anyway - it's also fairly disruptive >>> in that it uses stop_machine() to halt activity everywhere while taking >>> the CPU offline. >> >> We have a test that consists in shutting down all CPUs as frequently as >> we can and do this for about 2 million iterations which takes roughly >> 24h, and this printk slows thing down by a reasonable amount. Here are >> some numbers on 500 hotplug operations: >> >> w/ printk: >> real 0m9.997s >> user 0m0.725s >> sys 0m3.030s >> # >> >> w/o printk: >> real 0m8.547s >> user 0m0.436s >> sys 0m1.838s > > I am curious that a single printk() might make such a big difference.
It does, because of how printk() is implemented (there is nothing wrong with it, just slow by nature and how the UART gets written to as well).
> > One reason might be that the messages are pushed to a "slow" console.
115200 UART, yes that's slow, but not unusual.
> > Another reason might be that there are many other messages printed > on the system and there is a contention on logbuf_lock or other > console related locks.
The other messages being printed are those from the hotplug script that I run which just checkpoints its running every 50 instances, so it does not occur that often, the console really is not busy, which really extracts the overhead of printing "CPU: shutdown".
> > There might be also the opposite problem. The messages are also read > by userspace tools that store them into /var/log/messages or systemd > logs. If these are the only messages printed to the log and if there > is no other activity on the system. Then the waken loggers might make > a difference, especially if all CPUs are getting disabled and only > one is available at some point.
There is none of that, no systemd, no syslog, just minimal userspace running.
> > Well, I am not sure what other operations are needed to do the > CPU hotplug operation. > > I cannot judge how the message is useful and if the speed up > is worth removing it. > > Best Regards, > Petr >
-- Florian
| |