On Thu, Nov 21, 2024 at 11:27 PM Mario Limonciello <mario.limonciello@xxxxxxx> wrote: > > On 11/21/2024 07:15, Rafael J. Wysocki wrote: > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > > > > As stated by Len in [1], the extra delay added by msleep() to the > > sleep time value passed to it can be significant, roughly between > > 1.5 ns on systems with HZ = 1000 and as much as 15 ms on systems with > > HZ = 100, which is hardly acceptable, at least for small sleep time > > values. > > > > Address this by using usleep_range() in acpi_os_sleep() instead of > > msleep(). For short sleep times this is a no-brainer, but even for > > long sleeps usleep_range() should be preferred because timer wheel > > timers are optimized for cancellation before they expire and this > > particular timer is not going to be canceled. > > > > Add at least 50 us on top of the requested sleep time in case the > > timer can be subject to coalescing, which is consistent with what's > > done in user space in this context [2], but for sleeps longer than 5 ms > > use 1% of the requested sleep time for this purpose. > > > > The rationale here is that longer sleeps don't need that much of a timer > > precision as a rule and making the timer a more likely candidate for > > coalescing in these cases is generally desirable. It starts at 5 ms so > > that the delta between the requested sleep time and the effective > > deadline is a contiuous function of the former. > > > > Link: https://lore.kernel.org/linux-pm/c7db7e804c453629c116d508558eaf46477a2d73.1731708405.git.len.brown@xxxxxxxxx/ [1] > > Link: https://lore.kernel.org/linux-pm/CAJvTdK=Q1kwWA6Wxn8Zcf0OicDEk6cHYFAvQVizgA47mXu63+g@xxxxxxxxxxxxxx/ [2] > > Reported-by: Len Brown <lenb@xxxxxxxxxx> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > > You probably should also pick up this tag from the earlier version. > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=216263 Good point. > > --- > > > > This is a follow-up to the discussion started by [1] above and since > > the beginning of it I have changed my mind a bit, as you can see. > > > > Given Arjan's feedback, I've concluded that using usleep_range() for > > all sleep values is the right choice and that some slack should be > > used there. I've taken 50 us as the minimum value of it because that's > > what is used in user space FWICT and I'm not convinced that shorter > > values would be suitable here. > > > > The other part, using 1% of the sleep time as the slack for longer > > sleeps, is likely more controversial. It is roughly based on the > > observation that if one timer interrupt is sufficient for something, > > then using two of them will be wasteful even if this is just somewhat. > > > > Anyway, please let me know what you think. I'd rather do whatever > > the majority of you are comfortable with. > > Generally I'm fine with this. > > I'm about to head on US holiday, but I will forward this to folks that > aren't and get some testing input on it to bring back later when I'm back. Thanks!
