[PATCH] x86/resctrl: Fix incorrect local bandwidth when mba_sc is enabled

* [PATCH] x86/resctrl: Fix incorrect local bandwidth when mba_sc is enabled
@ 2020-12-04  6:27 Xiaochen Shen
  2020-12-09 19:06 ` [tip: x86/cache] " tip-bot2 for Xiaochen Shen
  2020-12-10 16:57 ` [tip: x86/urgent] " tip-bot2 for Xiaochen Shen
  0 siblings, 2 replies; 10+ messages in thread
From: Xiaochen Shen @ 2020-12-04  6:27 UTC (permalink / raw)
  To: tglx, mingo, bp, hpa, tony.luck, fenghua.yu, reinette.chatre
  Cc: x86, linux-kernel, pei.p.jia, xiaochen.shen

MBA software controller (mba_sc) is a feedback loop which periodically
reads MBM counters and tries to restrict the bandwidth below a user
specified bandwidth. It tags along MBM counter overflow handler to do
the updates with 1s interval in mbm_update() and update_mba_bw().

The purpose of mbm_update() is to periodically read the MBM counters to
make sure that the hardware counter doesn't wrap around more than once
between user samplings. mbm_update() calls __mon_event_count() for local
bandwidth updating when mba_sc is not enabled, but calls mbm_bw_count()
instead when mba_sc is enabled. __mon_event_count() will not be called
for local bandwidth updating in MBM counter overflow handler, but it is
still called when reading MBM local bandwidth counter file
'mbm_local_bytes', the call path is as below:

  rdtgroup_mondata_show()
    mon_event_read()
      mon_event_count()
        __mon_event_count()

In __mon_event_count(), m->chunks is updated by delta chunks which is
calculated from previous MSR value (m->prev_msr) and current MSR value.
When mba_sc is enabled, m->chunks is also updated in mbm_update() by
mistake by the delta chunks which is calculated from m->prev_bw_msr
instead of m->prev_msr. But m->chunks is not used in update_mba_bw() in
the mba_sc feedback loop.

When reading MBM local bandwidth counter file, m->chunks was changed
unexpectedly by mbm_bw_count(). As a result, the incorrect local
bandwidth counter which calculated from incorrect m->chunks is read out
to the user.

Fix this by removing incorrect m->chunks updating in mbm_bw_count() in
MBM counter overflow handler, and always calling __mon_event_count() in
mbm_update() to make sure that the hardware local bandwidth counter
doesn't wrap around.

Test steps:
  # Run workload with aggressive memory bandwidth (e.g., 10 GB/s)
  git clone https://github.com/intel/intel-cmt-cat && cd intel-cmt-cat
  && make
  ./tools/membw/membw -c 0 -b 10000 --read

  # Enable MBA software controller
  mount -t resctrl resctrl -o mba_MBps /sys/fs/resctrl

  # Create control group c1
  mkdir /sys/fs/resctrl/c1

  # Set MB throttle to 6 GB/s
  echo "MB:0=6000;1=6000" > /sys/fs/resctrl/c1/schemata

  # Write PID of the workload to tasks file
  echo `pidof membw` > /sys/fs/resctrl/c1/tasks

  # Read local bytes counters twice with 1s interval, the calculated
  # local bandwidth is not as expected (approaching to 6 GB/s):
  local_1=`cat /sys/fs/resctrl/c1/mon_data/mon_L3_00/mbm_local_bytes`
  sleep 1
  local_2=`cat /sys/fs/resctrl/c1/mon_data/mon_L3_00/mbm_local_bytes`
  echo "local b/w (bytes/s):" `expr $local_2 - $local_1`

Before fix:
  local b/w (bytes/s): 11076796416

After fix:
  local b/w (bytes/s): 5465014272

Fixes: ba0f26d8529c (x86/intel_rdt/mba_sc: Prepare for feedback loop)
Signed-off-by: Xiaochen Shen <xiaochen.shen@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
---
 arch/x86/kernel/cpu/resctrl/monitor.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 54dffe574e67..a98519a3a2e6 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -279,7 +279,6 @@ static void mbm_bw_count(u32 rmid, struct rmid_read *rr)
 		return;
 
 	chunks = mbm_overflow_count(m->prev_bw_msr, tval, rr->r->mbm_width);
-	m->chunks += chunks;
 	cur_bw = (chunks * r->mon_scale) >> 20;
 
 	if (m->delta_comp)
@@ -450,15 +449,14 @@ static void mbm_update(struct rdt_resource *r, struct rdt_domain *d, int rmid)
 	}
 	if (is_mbm_local_enabled()) {
 		rr.evtid = QOS_L3_MBM_LOCAL_EVENT_ID;
+		__mon_event_count(rmid, &rr);
 
 		/*
 		 * Call the MBA software controller only for the
 		 * control groups and when user has enabled
 		 * the software controller explicitly.
 		 */
-		if (!is_mba_sc(NULL))
-			__mon_event_count(rmid, &rr);
-		else
+		if (is_mba_sc(NULL))
 			mbm_bw_count(rmid, &rr);
 	}
 }
-- 
1.8.3.1


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