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.
