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Message-ID: <192aa189-8b08-c8c1-15dc-722e196493f4@intel.com>
Date: Wed, 9 Aug 2023 15:35:25 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: James Morse <james.morse@....com>, <x86@...nel.org>,
<linux-kernel@...r.kernel.org>
CC: Fenghua Yu <fenghua.yu@...el.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
H Peter Anvin <hpa@...or.com>,
Babu Moger <Babu.Moger@....com>,
<shameerali.kolothum.thodi@...wei.com>,
D Scott Phillips OS <scott@...amperecomputing.com>,
<carl@...amperecomputing.com>, <lcherian@...vell.com>,
<bobo.shaobowang@...wei.com>, <tan.shaopeng@...itsu.com>,
<xingxin.hx@...nanolis.org>, <baolin.wang@...ux.alibaba.com>,
Jamie Iles <quic_jiles@...cinc.com>,
Xin Hao <xhao@...ux.alibaba.com>, <peternewman@...gle.com>,
<dfustini@...libre.com>
Subject: Re: [PATCH v5 12/24] x86/resctrl: Make resctrl_arch_rmid_read() retry
when it is interrupted
Hi James,
On 7/28/2023 9:42 AM, James Morse wrote:
> 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.
The changelog is written as a bug report of current behavior.
This does not seem to describe current but instead planned future behavior.
> 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.
> Similarly __rmid_read() must write to one MSR and read from another,
> this must be proteted from re-entrance.
proteted -> protected
>
> Read the prev_msr before accessing the hardware, and cmpxchg() the value
> back. If the value has changed, the whole thing is re-attempted. To protect
> the MSR, __rmid_read() will retry reads for QM_CTR if QM_EVTSEL has changed
> from the selected value.
The latter part of the sentence does not seem to match with what the
patch does.
>
> Signed-off-by: James Morse <james.morse@....com>
>
> ---
> Changes since v4:
> * Added retry loop in __rmid_read() to protect the CPU MSRs.
> ---
> arch/x86/kernel/cpu/resctrl/internal.h | 5 +--
> arch/x86/kernel/cpu/resctrl/monitor.c | 45 ++++++++++++++++++++------
> 2 files changed, 38 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index a32d307292a1..7012f42a82ee 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 f0670795b446..62350bbd23e0 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -16,6 +16,7 @@
> */
>
> #include <linux/module.h>
> +#include <linux/percpu.h>
> #include <linux/sizes.h>
> #include <linux/slab.h>
>
> @@ -24,6 +25,9 @@
>
> #include "internal.h"
>
> +/* Sequence number for writes to IA32 QM_EVTSEL */
> +static DEFINE_PER_CPU(u64, qm_evtsel_seq);
> +
> struct rmid_entry {
> /*
> * Some architectures's resctrl_arch_rmid_read() needs the CLOSID value
> @@ -178,7 +182,7 @@ static inline struct rmid_entry *__rmid_entry(u32 idx)
>
> static int __rmid_read(u32 rmid, enum resctrl_event_id eventid, u64 *val)
> {
> - u64 msr_val;
> + u64 msr_val, seq;
>
> /*
> * As per the SDM, when IA32_QM_EVTSEL.EvtID (bits 7:0) is configured
> @@ -187,9 +191,16 @@ static int __rmid_read(u32 rmid, enum resctrl_event_id eventid, u64 *val)
> * IA32_QM_CTR.data (bits 61:0) reports the monitored data.
> * IA32_QM_CTR.Error (bit 63) and IA32_QM_CTR.Unavailable (bit 62)
> * are error bits.
> + * A per-cpu sequence counter is incremented each time QM_EVTSEL is
> + * written. This is used to detect if this function was interrupted by
> + * another call without re-reading the MSRs. Retry the MSR read when
> + * this happens as the QM_CTR value may belong to a different event.
> */
> - wrmsr(MSR_IA32_QM_EVTSEL, eventid, rmid);
> - rdmsrl(MSR_IA32_QM_CTR, msr_val);
> + do {
> + seq = this_cpu_inc_return(qm_evtsel_seq);
> + wrmsr(MSR_IA32_QM_EVTSEL, eventid, rmid);
> + rdmsrl(MSR_IA32_QM_CTR, msr_val);
> + } while (seq != this_cpu_read(qm_evtsel_seq));
>
> if (msr_val & RMID_VAL_ERROR)
> return -EIO;
> @@ -225,13 +236,15 @@ 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 +279,35 @@ 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;
> +
hmmm ... what if interruption occurs here?
> + 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;
> }
Reinette
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