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Message-ID: <5b4da95f-28ac-a571-6c01-cf1a94d0753d@arm.com>
Date:   Thu, 8 Sep 2022 18:00:38 +0100
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Subject: Re: [PATCH v6 12/21] x86/resctrl: Calculate bandwidth from the
 previous __mon_event_count() chunks
Content-Language: en-GB
To:     haoxin <xhao@linux.alibaba.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Fenghua Yu <fenghua.yu@intel.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Babu Moger <Babu.Moger@amd.com>,
        shameerali.kolothum.thodi@huawei.com,
        D Scott Phillips OS <scott@os.amperecomputing.com>,
        lcherian@marvell.com, bobo.shaobowang@huawei.com,
        tan.shaopeng@fujitsu.com, Jamie Iles <quic_jiles@quicinc.com>,
        Cristian Marussi <cristian.marussi@arm.com>,
        xingxin.hx@openanolis.org, baolin.wang@linux.alibaba.com
References: <20220902154829.30399-1-james.morse@arm.com>
 <20220902154829.30399-13-james.morse@arm.com>
 <87aa9fd2-cae8-972d-81d4-8855cbd81569@linux.alibaba.com>
From:   James Morse <james.morse@arm.com>
In-Reply-To: <87aa9fd2-cae8-972d-81d4-8855cbd81569@linux.alibaba.com>
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Precedence: bulk

Hi Hao Xin,

On 07/09/2022 07:47, haoxin wrote:
> 在 2022/9/2 下午11:48, James Morse 写道:
>> mbm_bw_count() is only called by the mbm_handle_overflow() worker once a
>> second. It reads the hardware register, calculates the bandwidth and
>> updates m->prev_bw_msr which is used to hold the previous hardware register
>> value.
>>
>> Operating directly on hardware register values makes it difficult to make
>> this code architecture independent, so that it can be moved to /fs/,
>> making the mba_sc feature something resctrl supports with no additional
>> support from the architecture.
>> Prior to calling mbm_bw_count(), mbm_update() reads from the same hardware
>> register using __mon_event_count().
>>
>> Change mbm_bw_count() to use the current chunks value most recently saved
>> by __mon_event_count(). This removes an extra call to __rmid_read().
>> Instead of using m->prev_msr to calculate the number of chunks seen,
>> use the rr->val that was updated by __mon_event_count(). This removes an
>> extra call to mbm_overflow_count() and get_corrected_mbm_count().
>> Calculating bandwidth like this means mbm_bw_count() no longer operates
>> on hardware register values directly.

>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>> index 3e69386cfe00..2d81b6cd9632 100644
>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c

>> @@ -516,10 +521,12 @@ static void mbm_update(struct rdt_resource *r, struct rdt_domain
>> *d, int rmid)
>>        */
>>       if (is_mbm_total_enabled()) {
>>           rr.evtid = QOS_L3_MBM_TOTAL_EVENT_ID;>> +        rr.val = 0;

> In mbm_update,  there no use the rr.val, so there no need to initialize ？

>>           __mon_event_count(rmid, &rr);
>>       }
>>       if (is_mbm_local_enabled()) {
>>           rr.evtid = QOS_L3_MBM_LOCAL_EVENT_ID;
>> +        rr.val = 0;

> ditto.

>>           __mon_event_count(rmid, &rr);
>>             /*

No, but this just leaves that problem for someone else to discover the hard way! I think
its fair for the compiler to complain that addition on an uninitialised field is a bug.

I'd prefer to keep this as it is on the principle of 'least surprise'.


Thanks,

James