Messages in this thread | | | Date | Thu, 27 May 2021 21:05:14 +0800 | From | Chen Yu <> | Subject | Re: [PATCH] intel_idle: Adjust the SKX C6 latency and residency if PC6 is disabled |
| |
On Thu, May 27, 2021 at 11:25:18AM +0300, Artem Bityutskiy wrote: > On Thu, 2021-05-27 at 12:56 +0800, Chen Yu wrote: > > ... snip ... > > > Exit latency: > > The C6 exit latency is measured when woken up from CC6/PC6. In the past, > > if PC6 is disabled, CPU would be demoted to CC6/PC3, which is close to > > the latency from CC6/PC6 and there is no need to update the C6 exit latency. > > However on newer platform there is no CC3/PC3 anymore, then the C6 exit > > latency with PC6 disabled should be CC6/PC0. > > > > Target residency: > > With PC6 disabled and C3/PC3 supported, the OS requests C3 if idle > > duration is within [CC6, PC6) target_residency. On new CPU generations > > with C3/PC3 deprecated, the OS would request C1E. This would cause > > low energy-efficiency. In summary, the question is, should we lower > > the bar to request C6 when PC6 is disabled? The answer is yes. > ... snip ... > > Hi Yu, > Hi Artem, > Thanks for this patch, it is very actual and helpful. > > Comments about the commit message below. > > This patch is specifically about SKX. It also covers CLX and CPX, > because they have the same ID. > > Now, this platforms do not have C3 and PC3. So I would avoid talking > about these states > in the commit message. Why making a simple thing more complex? > I agree, in theory this issue is irrelevant of the C3/PC3, however I was thinking the issue would become more significant to user on platform without C3/PC3, and this increase the necessity of this patch. But let me refine the commit log to only mention C3 a little just for future reference. > Here are all the SKX C-states. > > 1. Linux-level C-states (linux can ask for): C1, C1E, C6. > 2. HW-level C-states (HW supports under the hood): C1, C1E, CC6, PC2, > PC6. > And PC0? > Here is the story of this patch in my understanding. > > 1. C6 maps to CC6 and PC6. > 2. CC6 is "shallower" than PC6. > 3. Linux assumes worst case - PC6. > 4. Many datacenters and users disable PC6. > 5. We can optimize intel_idle in this case: adjust C6 latency and > target residency to match (faster) CC6. > > That's it. > Okay. > Then may be it is worth mentioning that CC6 vs PC2 difference is not > really measurable, so > the adjustment is only for PC6. > Okay.
thanks, Chenyu > Artem. >
| |