On Mon, 2020-01-13 at 11:16 +0100, Peter Zijlstra wrote: > On Fri, Jan 10, 2020 at 07:35:20AM -0800, Eduardo Valentin wrote: > > Hey Peter, > > > > On Wed, Jan 08, 2020 at 11:50:11AM +0100, Peter Zijlstra wrote: > > > On Tue, Jan 07, 2020 at 11:45:26PM +0000, Anchal Agarwal wrote: > > > > From: Eduardo Valentin <eduval@xxxxxxxxxx> > > > > > > > > System instability are seen during resume from hibernation when system > > > > is under heavy CPU load. This is due to the lack of update of sched > > > > clock data, and the scheduler would then think that heavy CPU hog > > > > tasks need more time in CPU, causing the system to freeze > > > > during the unfreezing of tasks. For example, threaded irqs, > > > > and kernel processes servicing network interface may be delayed > > > > for several tens of seconds, causing the system to be unreachable. > > > > The fix for this situation is to mark the sched clock as unstable > > > > as early as possible in the resume path, leaving it unstable > > > > for the duration of the resume process. This will force the > > > > scheduler to attempt to align the sched clock across CPUs using > > > > the delta with time of day, updating sched clock data. In a post > > > > hibernation event, we can then mark the sched clock as stable > > > > again, avoiding unnecessary syncs with time of day on systems > > > > in which TSC is reliable. > > > > > > This makes no frigging sense what so bloody ever. If the clock is > > > stable, we don't care about sched_clock_data. When it is stable you get > > > a linear function of the TSC without complicated bits on. > > > > > > When it is unstable, only then do we care about the sched_clock_data. > > > > > > > Yeah, maybe what is not clear here is that we covering for situation > > where clock stability changes over time, e.g. at regular boot clock is > > stable, hibernation happens, then restore happens in a non-stable clock. > > Still confused, who marks the thing unstable? The patch seems to suggest > you do yourself, but it is not at all clear why. > > If TSC really is unstable, then it needs to remain unstable. If the TSC > really is stable then there is no point in marking is unstable. > > Either way something is off, and you're not telling me what. > Hi, Peter For your original comment, just wanted to clarify the following: 1. After hibernation, the machine can be resumed on a different but compatible host (these are VM images hibernated) 2. This means the clock between host1 and host2 can/will be different In your comments are you making the assumption that the host(s) is/are the same? Just checking the assumptions being made and being on the same page with them. Balbir Singh. > > > > Reviewed-by: Erik Quanstrom <quanstro@xxxxxxxxxx> > > > > Reviewed-by: Frank van der Linden <fllinden@xxxxxxxxxx> > > > > Reviewed-by: Balbir Singh <sblbir@xxxxxxxxxx> > > > > Reviewed-by: Munehisa Kamata <kamatam@xxxxxxxxxx> > > > > Tested-by: Anchal Agarwal <anchalag@xxxxxxxxxx> > > > > Signed-off-by: Eduardo Valentin <eduval@xxxxxxxxxx> > > > > --- > > > > > > NAK, the code very much relies on never getting marked stable again > > > after it gets set to unstable. > > > > > > > Well actually, at the PM_POST_HIBERNATION, we do the check and set stable > > if > > known to be stable. > > > > The issue only really happens during the restoration path under scheduling > > pressure, > > which takes forever to finish, as described in the commit. > > > > Do you see a better solution for this issue? > > I still have no clue what your actual problem is. You say scheduling > goes wobbly because sched_clock_data is stale, but when stable that > doesn't matter. > > So what is the actual problem?
