Re: [PATCH v4 4/4] arm64: dts: qcom: sdm845: Add CPU BWMON

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Thu 23 Jun 07:58 CDT 2022, Krzysztof Kozlowski wrote:

> On 23/06/2022 08:48, Rajendra Nayak wrote:
> >>>> diff --git a/arch/arm64/boot/dts/qcom/sdm845.dtsi b/arch/arm64/boot/dts/qcom/sdm845.dtsi
> >>>> index 83e8b63f0910..adffb9c70566 100644
> >>>> --- a/arch/arm64/boot/dts/qcom/sdm845.dtsi
> >>>> +++ b/arch/arm64/boot/dts/qcom/sdm845.dtsi
> >>>> @@ -2026,6 +2026,60 @@ llcc: system-cache-controller@1100000 {
> >>>>    			interrupts = <GIC_SPI 582 IRQ_TYPE_LEVEL_HIGH>;
> >>>>    		};
> >>>>    
> >>>> +		pmu@1436400 {
> >>>> +			compatible = "qcom,sdm845-cpu-bwmon";
> >>>> +			reg = <0 0x01436400 0 0x600>;
> >>>> +
> >>>> +			interrupts = <GIC_SPI 581 IRQ_TYPE_LEVEL_HIGH>;
> >>>> +
> >>>> +			interconnects = <&gladiator_noc MASTER_APPSS_PROC 3 &mem_noc SLAVE_EBI1 3>,
> >>>> +					<&osm_l3 MASTER_OSM_L3_APPS &osm_l3 SLAVE_OSM_L3>;
> >>>> +			interconnect-names = "ddr", "l3c";
> >>>
> >>> Is this the pmu/bwmon instance between the cpu and caches or the one between the caches and DDR?
> >>
> >> To my understanding this is the one between CPU and caches.
> > 
> > Ok, but then because the OPP table lists the DDR bw first and Cache bw second, isn't the driver
> > ending up comparing the bw values thrown by the pmu against the DDR bw instead of the Cache BW?
> 
> I double checked now and you're right.
> 
> > Atleast with my testing on sc7280 I found this to mess things up and I always was ending up at
> > higher OPPs even while the system was completely idle. Comparing the values against the Cache bw
> > fixed it.(sc7280 also has a bwmon4 instance between the cpu and caches and a bwmon5 between the cache
> > and DDR)
> 
> In my case it exposes different issue - under performance. Somehow the
> bwmon does not report bandwidth high enough to vote for high bandwidth.
> 
> After removing the DDR interconnect and bandwidth OPP values I have for:
> sysbench --threads=8 --time=60 --memory-total-size=20T --test=memory
> --memory-block-size=4M run
> 
> 1. Vanilla: 29768 MB/s
> 2. Vanilla without CPU votes: 8728 MB/s
> 3. Previous bwmon (voting too high): 32007 MB/s
> 4. Fixed bwmon 24911 MB/s
> Bwmon does not vote for maximum L3 speed:
> bwmon report 9408 MB/s (thresholds set: <9216000 15052801>
> )
> osm l3 aggregate 14355 MBps -> 897 MHz, level 7, bw 14355 MBps
> 
> Maybe that's just problem with missing governor which would vote for
> bandwidth rounding up or anticipating higher needs.
> 
> >>> Depending on which one it is, shouldn;t we just be scaling either one and not both the interconnect paths?
> >>
> >> The interconnects are the same as ones used for CPU nodes, therefore if
> >> we want to scale both when scaling CPU, then we also want to scale both
> >> when seeing traffic between CPU and cache.
> > 
> > Well, they were both associated with the CPU node because with no other input to decide on _when_
> > to scale the caches and DDR, we just put a mapping table which simply mapped a CPU freq to a L3 _and_
> > DDR freq. So with just one input (CPU freq) we decided on what should be both the L3 freq and DDR freq.
> > 
> > Now with 2 pmu's, we have 2 inputs, so we can individually scale the L3 based on the cache PMU
> > counters and DDR based on the DDR PMU counters, no?
> > 
> > Since you said you have plans to add the other pmu support as well (bwmon5 between the cache and DDR)
> > how else would you have the OPP table associated with that pmu instance? Would you again have both the
> > L3 and DDR scale based on the inputs from that bwmon too?
> 
> Good point, thanks for sharing. I think you're right. I'll keep only the
> l3c interconnect path.
> 

If I understand correctly, <&osm_l3 MASTER_OSM_L3_APPS &osm_l3
SLAVE_OSM_L3> relates to the L3 cache speed, which sits inside the CPU
subsystem. As such traffic hitting this cache will not show up in either
bwmon instance.

The path <&gladiator_noc MASTER_APPSS_PROC 3 &mem_noc SLAVE_EBI1 3>
affects the DDR frequency. So the traffic measured by the cpu-bwmon
would be the CPU subsystems traffic that missed the L1/L2/L3 caches and
hits the memory bus towards DDR.


If this is the case it seems to make sense to keep the L3 scaling in the
opp-tables for the CPU and make bwmon only scale the DDR path. What do
you think?

Regards,
Bjorn