On Mon, Jan 21, 2019 at 03:23:11PM +0000, Patrick Bellasi wrote: > On 21-Jan 15:59, Peter Zijlstra wrote: > > On Tue, Jan 15, 2019 at 10:15:01AM +0000, Patrick Bellasi wrote: > > > @@ -835,6 +954,28 @@ static void uclamp_bucket_inc(struct uclamp_se *uc_se, unsigned int clamp_id, > > > } while (!atomic_long_try_cmpxchg(&uc_maps[bucket_id].adata, > > > &uc_map_old.data, uc_map_new.data)); > > > > > > + /* > > > + * Ensure each CPU tracks the correct value for this clamp bucket. > > > + * This initialization of per-CPU variables is required only when a > > > + * clamp value is requested for the first time from a slow-path. > > > + */ > > > > I'm confused; why is this needed? > > That's a lazy initialization of the per-CPU uclamp data for a given > bucket, i.e. the clamp value assigned to a bucket, which happens only > when new clamp values are requested... usually only at system > boot/configuration time. > > For example, let say we have these buckets mapped to given clamp > values: > > bucket_#0: clamp value: 10% (mapped) > bucket_#1: clamp value: 20% (mapped) > bucket_#2: clamp value: 30% (mapped) > > and then let's assume all the users of bucket_#1 are "destroyed", i.e. > there are no more tasks, system defaults or cgroups asking for a > 20% clamp value. The corresponding bucket will become free: > > bucket_#0: clamp value: 10% (mapped) > bucket_#1: clamp value: 20% (free) > bucket_#2: clamp value: 30% (mapped) > > If, in the future, we ask for a new clamp value, let say a task ask > for a 40% clamp value, then we need to map that value into a bucket. > Since bucket_#1 is free we can use it to fill up the hold and keep all > the buckets in use at the beginning of a cache line. > > However, since now bucket_#1 tracks a different clamp value (40 > instead of 20) we need to walk all the CPUs and updated the cached > value: > > bucket_#0: clamp value: 10% (mapped) > bucket_#1: clamp value: 40% (mapped) > bucket_#2: clamp value: 30% (mapped) > > Is that more clear ? Yes, and I realized this a little while after sending this; but I'm not sure I have an answer to why though. That is; why isn't the whole thing hard coded to have: bucket_n: clamp value: n*UCLAMP_BUCKET_DELTA We already do that division anyway (clamp_value / UCLAMP_BUCKET_DELTA), and from that we instantly have the right bucket index. And that allows us to initialize all this beforehand. > and keep all > the buckets in use at the beginning of a cache line. That; is that the rationale for all this? Note that per the defaults everything is in a single line already.
