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Date:   Tue, 28 Jun 2022 16:02:31 +0200
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Subject: Re: [PATCH v4 4/4] arm64: dts: qcom: sdm845: Add CPU BWMON
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To:     Rajendra Nayak <quic_rjendra@quicinc.com>,
        Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     Andy Gross <agross@kernel.org>, Georgi Djakov <djakov@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, linux-arm-msm@vger.kernel.org,
        linux-pm@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Thara Gopinath <thara.gopinath@linaro.org>
References: <20220601101140.170504-1-krzysztof.kozlowski@linaro.org>
 <20220601101140.170504-5-krzysztof.kozlowski@linaro.org>
 <bc423d7b-df03-d4e2-2898-0873db710943@quicinc.com>
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 <47e1fcb4-237b-b880-b1b2-3910cc19e727@linaro.org>
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 <173717b1-9cd9-9298-3a20-7dd3d95339af@quicinc.com>
From:   Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
In-Reply-To: <173717b1-9cd9-9298-3a20-7dd3d95339af@quicinc.com>
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On 28/06/2022 15:15, Rajendra Nayak wrote:
> 
> 
> On 6/28/2022 4:20 PM, Krzysztof Kozlowski wrote:
>> 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@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.
>>>
>>> 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)

For sdm845 SLAVE_LLCC is in mem_noc, so I guess mc_virt on sc7280?

>>> 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.
> 
> Right, thats because the bandwidth supported between the cpu<->llcc path is much higher
> than the DDR frequencies. For instance on sc7280, I see (2288 - 15258 MB/s) for LLCC while
> the DDR max is 8532 MB/s.

OK, that sounds right.

Another point is that I did not find actual scaling of throughput via
that interconnect path:
<&gladiator_noc MASTER_APPSS_PROC 3 &mem_noc SLAVE_LLCC 3>

so I cannot test impact of bwmon that way.

Best regards,
Krzysztof