Re: Alternatives to /sys/kernel/debug/wakeup_sources

[Joel Fernandes]

On Wed, Jun 19, 2019 at 3:59 PM 'Saravana Kannan' via kernel-team
<kernel-team@xxxxxxxxxxx> wrote:
>
>
>
> On Wed, Jun 19, 2019, 11:55 AM 'Joel Fernandes' via kernel-team <kernel-team@xxxxxxxxxxx> wrote:
>>
>> On Wed, Jun 19, 2019 at 2:35 PM Greg Kroah-Hartman
>> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> >
>> > On Wed, Jun 19, 2019 at 02:01:36PM -0400, Joel Fernandes wrote:
>> > > On Wed, Jun 19, 2019 at 1:07 PM Greg Kroah-Hartman
>> > > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> > > >
>> > > > On Wed, Jun 19, 2019 at 12:53:12PM -0400, Joel Fernandes wrote:
>> > > > > > It is conceivable to have a "wakeup_sources" directory under
>> > > > > > /sys/power/ and sysfs nodes for all wakeup sources in there.
>> > > > >
>> > > > > One of the "issues" with this is, now if you have say 100 wake up
>> > > > > sources, with 10 entries each, then we're talking about a 1000 sysfs
>> > > > > files. Each one has to be opened, and read individually. This adds
>> > > > > overhead and it is more convenient to read from a single file. The
>> > > > > problem is this single file is not ABI. So the question I guess is,
>> > > > > how do we solve this in both an ABI friendly way while keeping the
>> > > > > overhead low.
>> > > >
>> > > > How much overhead?  Have you measured it, reading from virtual files is
>> > > > fast :)
>> > >
>> > > I measured, and it is definitely not free. If you create and read a
>> > > 1000 files and just return a string back, it can take up to 11-13
>> > > milliseconds (did not lock CPU frequencies, was just looking for
>> > > average ball park). This is assuming that the counter reading is just
>> > > doing that, and nothing else is being done to return the sysfs data
>> > > which is probably not always true in practice.
>> > >
>> > > Our display pipeline deadline is around 16ms at 60Hz. Conceivably, any
>> > > CPU scheduling competion reading sysfs can hurt the deadline. There's
>> > > also the question of power - we definitely have spent time in the past
>> > > optimizing other virtual files such as /proc/pid/smaps for this reason
>> > > where it spent lots of CPU time.
>> >
>> > smaps was "odd", but that was done after measurements were actually made
>> > to prove it was needed.  That hasn't happened yet :)
>> >
>> > And is there a reason you have to do this every 16ms?
>>
>> Not every, I was just saying whenever it happens and a frame delivery
>> deadline is missed, then a frame drop can occur which can result in a
>> poor user experience.
>
>
> But this is not done in the UI thread context. So some thread running for more than 16ms shouldn't cause a frame drop. If it does, we have bigger problems.
>

Not really. That depends on the priority of the other thread and other
things. It can obviously time share the same CPU as the UI thread if
it is not configured correctly. Even with CFS it can reduce the time
consumed by other "real-time" CFS threads. I am not sure what you are
proposing, there are also (obviously) power issues with things running
for long times pointlessly. We should try to do better if we can. As
Greg said, some study/research can be done on the use case before
settling for a solution (sysfs or other).