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Subject[PATCH v4 12/24] x86/resctrl: Make resctrl_arch_rmid_read() retry when it is interrupted
Date
resctrl_arch_rmid_read() could be called by resctrl in process context,
and then called by the PMU driver from irq context on the same CPU.
This could cause struct arch_mbm_state's prev_msr value to go backwards,
leading to the chunks value being incremented multiple times.

The struct arch_mbm_state holds both the previous msr value, and a count
of the number of chunks. These two fields need to be updated atomically.

Read the prev_msr before accessing the hardware, and cmpxchg() the value
back. If the value has changed, the whole thing is re-attempted.

Signed-off-by: James Morse <james.morse@arm.com>
---
arch/x86/kernel/cpu/resctrl/internal.h | 5 +++--
arch/x86/kernel/cpu/resctrl/monitor.c | 28 +++++++++++++++++++-------
2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
index 6f18cf26988c..7960366b9434 100644
--- a/arch/x86/kernel/cpu/resctrl/internal.h
+++ b/arch/x86/kernel/cpu/resctrl/internal.h
@@ -2,6 +2,7 @@
#ifndef _ASM_X86_RESCTRL_INTERNAL_H
#define _ASM_X86_RESCTRL_INTERNAL_H

+#include <linux/atomic.h>
#include <linux/resctrl.h>
#include <linux/sched.h>
#include <linux/kernfs.h>
@@ -338,8 +339,8 @@ struct mbm_state {
* find this struct.
*/
struct arch_mbm_state {
- u64 chunks;
- u64 prev_msr;
+ atomic64_t chunks;
+ atomic64_t prev_msr;
};

/**
diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index e267869d60d5..1f470e55d555 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -225,13 +225,15 @@ void resctrl_arch_reset_rmid(struct rdt_resource *r, struct rdt_domain *d,
{
struct rdt_hw_domain *hw_dom = resctrl_to_arch_dom(d);
struct arch_mbm_state *am;
+ u64 msr_val;

am = get_arch_mbm_state(hw_dom, rmid, eventid);
if (am) {
memset(am, 0, sizeof(*am));

/* Record any initial, non-zero count value. */
- __rmid_read(rmid, eventid, &am->prev_msr);
+ __rmid_read(rmid, eventid, &msr_val);
+ atomic64_set(&am->prev_msr, msr_val);
}
}

@@ -266,23 +268,35 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
{
struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
struct rdt_hw_domain *hw_dom = resctrl_to_arch_dom(d);
+ u64 start_msr_val, old_msr_val, msr_val, chunks;
struct arch_mbm_state *am;
- u64 msr_val, chunks;
- int ret;
+ int ret = 0;

if (!cpumask_test_cpu(smp_processor_id(), &d->cpu_mask))
return -EINVAL;

+interrupted:
+ am = get_arch_mbm_state(hw_dom, rmid, eventid);
+ if (am)
+ start_msr_val = atomic64_read(&am->prev_msr);
+
ret = __rmid_read(rmid, eventid, &msr_val);
if (ret)
return ret;

am = get_arch_mbm_state(hw_dom, rmid, eventid);
if (am) {
- am->chunks += mbm_overflow_count(am->prev_msr, msr_val,
- hw_res->mbm_width);
- chunks = get_corrected_mbm_count(rmid, am->chunks);
- am->prev_msr = msr_val;
+ old_msr_val = atomic64_cmpxchg(&am->prev_msr, start_msr_val,
+ msr_val);
+ if (old_msr_val != start_msr_val)
+ goto interrupted;
+
+ chunks = mbm_overflow_count(start_msr_val, msr_val,
+ hw_res->mbm_width);
+ atomic64_add(chunks, &am->chunks);
+
+ chunks = get_corrected_mbm_count(rmid,
+ atomic64_read(&am->chunks));
} else {
chunks = msr_val;
}
--
2.39.2
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