linux-kernel - RE: [PATCH v4 01/13] x86/resctrl: Fix min_cbm

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Message-ID: <MW3PR12MB4553557C746E9B432AA5935195489@MW3PR12MB4553.namprd12.prod.outlook.com>
Date:   Fri, 16 Sep 2022 18:31:05 +0000
From:   "Moger, Babu" <Babu.Moger@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        "corbet@....net" <corbet@....net>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
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        "bp@...en8.de" <bp@...en8.de>
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        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
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Subject: RE: [PATCH v4 01/13] x86/resctrl: Fix min_cbm_bits for AMD

[AMD Official Use Only - General]



> -----Original Message-----
> From: Reinette Chatre <reinette.chatre@...el.com>
> Sent: Friday, September 16, 2022 10:53 AM
> To: Moger, Babu <Babu.Moger@....com>; corbet@....net;
> tglx@...utronix.de; mingo@...hat.com; bp@...en8.de
> Cc: fenghua.yu@...el.com; dave.hansen@...ux.intel.com; x86@...nel.org;
> hpa@...or.com; paulmck@...nel.org; akpm@...ux-foundation.org;
> quic_neeraju@...cinc.com; rdunlap@...radead.org;
> damien.lemoal@...nsource.wdc.com; songmuchun@...edance.com;
> peterz@...radead.org; jpoimboe@...nel.org; pbonzini@...hat.com;
> chang.seok.bae@...el.com; pawan.kumar.gupta@...ux.intel.com;
> jmattson@...gle.com; daniel.sneddon@...ux.intel.com; Das1, Sandipan
> <Sandipan.Das@....com>; tony.luck@...el.com; james.morse@....com;
> linux-doc@...r.kernel.org; linux-kernel@...r.kernel.org;
> bagasdotme@...il.com; eranian@...gle.com
> Subject: Re: [PATCH v4 01/13] x86/resctrl: Fix min_cbm_bits for AMD
> 
> Hi Babu,
> 
> On 9/7/2022 10:59 AM, Babu Moger wrote:
> > AMD systems support zero CBM (capacity bit mask) for L3 allocation.
> 
> Above mentions "for L3 allocation", but the change impacts
> L3 as well as L2 allocation. Perhaps just "cache allocation"?
> 
> > That is reflected in rdt_init_res_defs_amd() by:
> >
> > 	r->cache.arch_has_empty_bitmaps = true;
> >
> > However given the unified code in cbm_validate(), checking for:
> > 	val == 0 && !arch_has_empty_bitmaps
> >
> > is not enough because of another check in cbm_validate():
> >
> > 	if ((zero_bit - first_bit) < r->cache.min_cbm_bits)
> >
> > The default value of r->cache.min_cbm_bits = 1.
> >
> > Leading to:
> >
> > 	$ cd /sys/fs/resctrl
> > 	$ mkdir foo
> > 	$ cd foo
> > 	$ echo L3:0=0 > schemata
> >           -bash: echo: write error: Invalid argument
> > 	$ cat /sys/fs/resctrl/info/last_cmd_status
> > 	  Need at least 1 bits in the mask
> >
> > Fix the issue by initializing the min_cbm_bits to 0 for AMD. Also,
> > remove the default setting of min_cbm_bits and initialize it separately.
> >
> > After the fix
> > 	$ cd /sys/fs/resctrl
> > 	$ mkdir foo
> > 	$ cd foo
> > 	$ echo L3:0=0 > schemata
> > 	$ cat /sys/fs/resctrl/info/last_cmd_status
> > 	  ok
> >
> > Link:
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore
> > .kernel.org%2Flkml%2F20220517001234.3137157-1-
> eranian%40google.com%2F&
> >
> amp;data=05%7C01%7Cbabu.moger%40amd.com%7Cfdccc20e6a234fb3872a0
> 8da97fb
> >
> 94fc%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C6379894040498
> 44076%7
> >
> CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI
> 6Ik1
> >
> haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=DIpkBAkI7lAjj5QQ4
> 5VahssT
> > dWGj%2FcUwGJDiHXNYzz8%3D&amp;reserved=0
> > Fixes: 316e7f901f5a ("x86/resctrl: Add struct
> > rdt_cache::arch_has_{sparse, empty}_bitmaps")
> > Signed-off-by: Stephane Eranian <eranian@...gle.com>
> > Signed-off-by: Babu Moger <babu.moger@....com>
> > Reviewed-by: Ingo Molnar <mingo@...nel.org>
> 
> (apart from the changelog nitpick)
> 
> Thank you for clarifying the way forward for this fix.
> 
> Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Thank you
Babu Moger