The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 06c5fe9b12dde1b62821f302f177c972bb1c81f9 Gitweb: https://git.kernel.org/tip/06c5fe9b12dde1b62821f302f177c972bb1c81f9 Author: Xiaochen Shen xiaochen.shen@intel.com AuthorDate: Fri, 04 Dec 2020 14:27:59 +08:00 Committer: Borislav Petkov bp@suse.de CommitterDate: Thu, 10 Dec 2020 17:52:37 +01:00
x86/resctrl: Fix incorrect local bandwidth when mba_sc is enabled
The MBA software controller (mba_sc) is a feedback loop which periodically reads MBM counters and tries to restrict the bandwidth below a user-specified value. It tags along the 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 shown 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 Signed-off-by: Borislav Petkov bp@suse.de Reviewed-by: Tony Luck tony.luck@intel.com Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/1607063279-19437-1-git-send-email-xiaochen.shen@in... --- 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 54dffe5..a98519a 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); } }