On Tue, 2020-04-28 at 09:03 +0200, Rasmus Villemoes wrote:
> On 23/01/2020 21.39, Steven Rostedt wrote:
> > 4.19.94-rt39-rc2 stable review patch.
> > If anyone has any objections, please let me know.
> >
> > ------------------
> >
> > From: Julien Grall <julien.grall@xxxxxxx>
> >
> > [ Upstream commit cef1b87f98823af923a386f3f69149acb212d4a1 ]
> >
> > As tglx puts it:
> > > If base == migration_base then there is no point to lock
> > > soft_expiry_lock
> > > simply because the timer is not executing the callback in soft
> > > irq context
> > > and the whole lock/unlock dance can be avoided.
>
> Hold on a second. This patch (hrtimer: Prevent using
> hrtimer_grab_expiry_lock() on migration_base) indeed seems to
> implement
> the optimization implied by the above, namely avoid the lock/unlock
> in
> case base == migration_base:
>
> > - if (timer->is_soft && base && base->cpu_base) {
> > + if (timer->is_soft && base != &migration_base) {
>
> But the followup patch (hrtimer: Add a missing bracket and hide
> `migration_base on !SMP) to fix the build on !SMP [the missing
> bracket
> part seems to have been fixed when backporting the above to 4.19-rt]
> replaces that logic by
>
> +static inline bool is_migration_base(struct hrtimer_clock_base
> *base)
> +{
> + return base == &migration_base;
> +}
> +
> ...
> - if (timer->is_soft && base != &migration_base) {
> + if (timer->is_soft && is_migration_base(base)) {
>
> in the SMP case, i.e. the exact opposite condition. One of these
> can't
> be correct.
>
> Assuming the followup patch was wrong and the condition should have
> read
>
> timer->is_soft && !is_migration_base(base)
>
> while keeping is_migration_base() false on !SMP might explain the
> problem I see. But I'd like someone who knows this code to chime in.
>
I don't know this code, but I think you're correct - the followup patch
reversed the condition by forgetting the !.
So, does your problem go away when you make that change?
Tom
> Thanks,
> Rasmus
