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

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Message-ID: <c43d4c9e-54d6-7b00-0ff7-e114850302ad@linaro.org>
Date:   Tue, 28 Jun 2022 12:50:06 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Rajendra Nayak <quic_rjendra@...cinc.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Andy Gross <agross@...nel.org>, Georgi Djakov <djakov@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, linux-arm-msm@...r.kernel.org,
        linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Thara Gopinath <thara.gopinath@...aro.org>
Subject: Re: [PATCH v4 4/4] arm64: dts: qcom: sdm845: Add CPU BWMON

On 28/06/2022 12:36, Rajendra Nayak wrote:
> 
> On 6/27/2022 6:09 PM, Krzysztof Kozlowski wrote:
>> On 26/06/2022 05:28, Bjorn Andersson wrote:
>>> 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@...0000 {
>>>>>>>>     			interrupts = <GIC_SPI 582 IRQ_TYPE_LEVEL_HIGH>;
>>>>>>>>     		};
>>>>>>>>     
>>>>>>>> +		pmu@...6400 {
>>>>>>>> +			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.
> 
> That seems right, looking some more into the downstream code and register definitions
> I see the 2 bwmon instances actually lie on the path outside CPU SS towards DDR,
> first one (bwmon4) is between the CPUSS and LLCC (system cache) and the second one
> (bwmon5) between LLCC and DDR. So we should use the counters from bwmon4 to
> scale the CPU-LLCC path (and not L3), on sc7280 that would mean splitting the
> <&gem_noc MASTER_APPSS_PROC 3 &mc_virt SLAVE_EBI1 3> into
> <&gem_noc MASTER_APPSS_PROC 3 &gem_noc SLAVE_LLCC 3> (voting based on the bwmon4 inputs)
> and <&mc_virt MASTER_LLCC 3 &mc_virt SLAVE_EBI1 3> (voting based on the bwmon5 inputs)
> and similar for sdm845 too.
> 
> L3 should perhaps still be voted based on the cpu freq as done today.

This would mean that original bandwidth values (800 - 7216 MB/s) were
correct. However we have still your observation that bwmon kicks in very
fast and my measurements that sampled bwmon data shows bandwidth ~20000
MB/s.


Best regards,
Krzysztof