linux-kernel - [PATCH] x86/resctrl: Do not round up mba

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Message-ID: <20240927223038.41198-1-tony.luck@intel.com>
Date: Fri, 27 Sep 2024 15:30:38 -0700
From: Tony Luck <tony.luck@...el.com>
To: Fenghua Yu <fenghua.yu@...el.com>,
	Reinette Chatre <reinette.chatre@...el.com>,
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
	Peter Newman <peternewman@...gle.com>,
	James Morse <james.morse@....com>,
	Babu Moger <babu.moger@....com>,
	Drew Fustini <dfustini@...libre.com>,
	Dave Martin <Dave.Martin@....com>,
	Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org,
	linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev,
	Tony Luck <tony.luck@...el.com>
Subject: [PATCH] x86/resctrl: Do not round up mba_MBps values in schemata file

When the mba_MBps mount option is used Linux initializes the
MBA control values in the schemata file to MBA_MAX_MBPS (which
is defined as U32_MAX). So the schemata file contains this line:

 MB:0=4294967295;1=4294967295

If a user edits the schemata file with vi(1) (or other editor) and
simply writes that line back, it gets changed to:

  MB:0=   4;1=   4

which sets maximum bandwidth to a very low value.

This happens because bw_validate() unconditionally rounds user supplied
values up to next multiple of r->membw.bw_gran. That results in a value
that will not fit into d->mbps_val[closid].

Rounding up only makes sense when mba_MBps is not enabled and control
values are expressed as percentage values.

Skip the roundup() when mba_MBps is enabled.

Signed-off-by: Tony Luck <tony.luck@...el.com>
---
 arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
index 50fa1fe9a073..ac636366c0cb 100644
--- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
+++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
@@ -55,7 +55,7 @@ static bool bw_validate(char *buf, unsigned long *data, struct rdt_resource *r)
 		return false;
 	}

-	*data = roundup(bw, (unsigned long)r->membw.bw_gran);
+	*data = is_mba_sc(r) ? bw : roundup(bw, (unsigned long)r->membw.bw_gran);
 	return true;
 }

-- 
2.46.1