On Wed, Jun 19, 2019 at 1:09 PM 'Joel Fernandes' via kernel-team <kernel-t...@android.com> wrote: > > On Wed, Jun 19, 2019 at 3:59 PM 'Saravana Kannan' via kernel-team > <kernel-t...@android.com> wrote: > > > > > > > > On Wed, Jun 19, 2019, 11:55 AM 'Joel Fernandes' via kernel-team > > <kernel-t...@android.com> wrote: > >> > >> On Wed, Jun 19, 2019 at 2:35 PM Greg Kroah-Hartman > >> <gre...@linuxfoundation.org> 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 > >> > > <gre...@linuxfoundation.org> 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). >
Agree, power and optimization is good. Just saying that the UI example is not a real one. If the UI thread is that poorly configured that some thread running for a second can cause frame drops in a multicore system, that's a problem with the UI framework design. -Saravana > -- > To unsubscribe from this group and stop receiving emails from it, send an > email to kernel-team+unsubscr...@android.com. >
