From: Babu Moger <babu.moger@amd.com>
To: Reinette Chatre <reinette.chatre@intel.com>,
fenghua.yu@intel.com, tglx@linutronix.de, mingo@redhat.com,
bp@alien8.de, x86@kernel.org, hpa@zytor.com,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/resctrl: Fix memory bandwidth counter width for AMD
Date: Tue, 2 Jun 2020 12:33:38 -0500 [thread overview]
Message-ID: <05e11ddd-ca0b-e454-0152-6c83e7b7d719@amd.com> (raw)
In-Reply-To: <63f23edd-39d3-0577-6e00-21066ddfe57f@intel.com>
On 6/2/20 12:13 PM, Reinette Chatre wrote:
> Hi Babu,
>
> On 6/1/2020 4:00 PM, Babu Moger wrote:
>> Memory bandwidth is calculated reading the monitoring counter
>> at two intervals and calculating the delta. It is the software’s
>> responsibility to read the count often enough to avoid having
>> the count roll over _twice_ between reads.
>>
>> The current code hardcodes the bandwidth monitoring counter's width
>> to 24 bits for AMD. This is due to default base counter width which
>> is 24. Currently, AMD does not implement the CPUID 0xF.[ECX=1]:EAX
>> to adjust the counter width. But, the AMD hardware supports much
>> wider bandwidth counter with the default width of 44 bits.
>>
>> Kernel reads these monitoring counters every 1 second and adjusts the
>> counter value for overflow. With 24 bits and scale value of 64 for AMD,
>> it can only measure up to 1GB/s without overflowing. For the rates
>> above 1GB/s this will fail to measure the bandwidth.
>>
>> Fix the issue setting the default width to 44 bits by adjusting the
>> offset.
>>
>> AMD future products will implement the CPUID 0xF.[ECX=1]:EAX.
>>
>> Signed-off-by: Babu Moger <babu.moger@amd.com>
>
> There is no fixes tag but if I understand correctly this issue has been
> present since AMD support was added to resctrl. This fix builds on top
> of a recent feature addition and would thus not work for earlier
> kernels. Are you planning to create a different fix for earlier kernels?
Yes. This was there from day one. I am going to back port to older kernels
once we arrive on the final patch. Do we need fixes tag here?
next prev parent reply other threads:[~2020-06-02 17:33 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-01 23:00 [PATCH] x86/resctrl: Fix memory bandwidth counter width for AMD Babu Moger
2020-06-01 23:23 ` Fenghua Yu
2020-06-02 14:30 ` Babu Moger
2020-06-02 17:13 ` Reinette Chatre
2020-06-02 17:33 ` Babu Moger [this message]
2020-06-02 21:59 ` Reinette Chatre
2020-06-02 21:51 ` Reinette Chatre
2020-06-02 22:12 ` Babu Moger
2020-06-02 23:28 ` Reinette Chatre
2020-06-03 15:04 ` Babu Moger
2020-07-01 22:13 Babu Moger
2020-07-07 14:04 ` Greg KH
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