Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 5 Sep 2018 09:36:42 +0200 | Subject | Re: [RFC PATCH 3/4] sched/topology: remove smt_gain |
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On Tue, 4 Sep 2018 at 12:37, Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote: > > * Vincent Guittot <vincent.guittot@linaro.org> [2018-09-04 11:36:26]: > > > Hi Srikar, > > > > Le Tuesday 04 Sep 2018 à 01:24:24 (-0700), Srikar Dronamraju a écrit : > > > However after this change, capacity_orig of each SMT thread would be > > > 1024. For example SMT 8 core capacity_orig would now be 8192. > > > > > > smt_gain was suppose to make a multi threaded core was slightly more > > > powerful than a single threaded core. I suspect if that sometimes hurt > > > > Is there system with both single threaded and multi threaded core ? > > That was the main open point for me (and for Qais too) > > > > I dont know of any systems that have come with single threaded and > multithreaded. However some user can still offline few threads in a core > while leaving other cores untouched. I dont really know why somebody > would want to do it. For example, some customer was toying with SMT 3 > mode in a SMT 8 power8 box.
In this case, it means that we have the same core capacity whatever the number of CPUs and a core with SMT 3 will be set with the same compute capacity as the core with SMT 8. Does it still make sense ?
> > > > > > us when doing load balance between 2 cores i.e at MC or DIE sched > > > domain. Even with 2 threads running on a core, the core might look > > > lightly loaded 2048/8192. Hence might dissuade movement to a idle core. > > > > Then, there is the sibling flag at SMT level that normally ensures 1 task per > > core for such UC > > > > Agree. > > > > > > > I always wonder why arch_scale_cpu_capacity() is called with NULL > > > sched_domain, in scale_rt_capacity(). This way capacity might actually > > > > Probably because until this v4.19-rcxx version, the rt scaling was done > > relatively to local cpu capacity: > > capacity = arch_scale_cpu() * scale_rt_capacity / SCHED_CAPACITY_SCALE > > > > Whereas now, it directly returns the remaining capacity > > > > > be more than the capacity_orig. I am always under an impression that > > > capacity_orig > capacity. Or am I misunderstanding that? > > > > You are right, there is a bug for SMT and the patch below should fix it. > > Nevertheless, we still have the problem in some other places in the code. > > > > Subject: [PATCH] sched/fair: fix scale_rt_capacity() for SMT > > > > Since commit: > > commit 523e979d3164 ("sched/core: Use PELT for scale_rt_capacity()") > > scale_rt_capacity() returns the remaining capacity and not a scale factor > > to apply on cpu_capacity_orig. arch_scale_cpu() is directly called by > > scale_rt_capacity() so we must take the sched_domain argument > > > > Fixes: 523e979d3164 ("sched/core: Use PELT for scale_rt_capacity()") > > Reported-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com> > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com> > > -- > Thanks and Regards > Srikar Dronamraju >
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