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Date: Tue, 7 Jun 2022 15:08:16 -0700
From: Fenghua Yu <fenghua.yu@...el.com>
To: James Morse <james.morse@....com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
Reinette Chatre <reinette.chatre@...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>,
lcherian@...vell.com, bobo.shaobowang@...wei.com,
tan.shaopeng@...itsu.com, Jamie Iles <quic_jiles@...cinc.com>,
Cristian Marussi <cristian.marussi@....com>,
Xin Hao <xhao@...ux.alibaba.com>, xingxin.hx@...nanolis.org,
baolin.wang@...ux.alibaba.com
Subject: Re: [PATCH v4 19/21] x86/resctrl: Rename and change the units of
resctrl_cqm_threshold
Hi, James,
On Tue, Apr 12, 2022 at 12:44:17PM +0000, James Morse wrote:
> resctrl_cqm_threshold is stored in a hardware specific chunk size,
> but exposed to user-space as bytes.
>
> This means the filesystem parts of resctrl need to know how the hardware
> counts, to convert the user provided byte value to chunks. The interface
> between the architecture's resctrl code and the filesystem ought to
> treat everything as bytes.
>
> Change the unit of resctrl_cqm_threshold to bytes. resctrl_arch_rmid_read()
> still returns its value in chunks, so this needs converting to bytes.
> As all the users have been touched, rename the variable to
> resctrl_rmid_realloc_threshold, which describes what the value is for.
>
> Neither r->num_rmid nor hw_res->mon_scale are guaranteed to be a power
> of 2, so the existing code introduces a rounding error from resctrl's
> theoretical fraction of the cache usage. This behaviour is kept as it
> ensures the user visible value matches the value read from hardware
> when the rmid will be reallocated.
>
> Reviewed-by: Jamie Iles <quic_jiles@...cinc.com>
> Tested-by: Xin Hao <xhao@...ux.alibaba.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@...itsu.com>
> Tested-by: Shaopeng Tan <tan.shaopeng@...itsu.com>
> Tested-by: Cristian Marussi <cristian.marussi@....com>
> Signed-off-by: James Morse <james.morse@....com>
> ---
> Changes since v3:
> * Preserved the rounding errors.
> ---
> arch/x86/kernel/cpu/resctrl/internal.h | 1 -
> arch/x86/kernel/cpu/resctrl/monitor.c | 41 +++++++++++++++-----------
> arch/x86/kernel/cpu/resctrl/rdtgroup.c | 9 ++----
> include/linux/resctrl.h | 2 ++
> 4 files changed, 28 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index bdb55c2fbdd3..c05e9b7cf77a 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -98,7 +98,6 @@ struct rmid_read {
> u64 val;
> };
>
> -extern unsigned int resctrl_cqm_threshold;
> extern bool rdt_alloc_capable;
> extern bool rdt_mon_capable;
> extern unsigned int rdt_mon_features;
> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 88988de0c96c..00f6e27e4e0d 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -37,8 +37,8 @@ static LIST_HEAD(rmid_free_lru);
> * @rmid_limbo_count count of currently unused but (potentially)
> * dirty RMIDs.
> * This counts RMIDs that no one is currently using but that
> - * may have a occupancy value > intel_cqm_threshold. User can change
> - * the threshold occupancy value.
> + * may have a occupancy value > resctrl_rmid_realloc_threshold. User can
> + * change the threshold occupancy value.
> */
> static unsigned int rmid_limbo_count;
>
> @@ -59,10 +59,10 @@ bool rdt_mon_capable;
> unsigned int rdt_mon_features;
>
> /*
> - * This is the threshold cache occupancy at which we will consider an
> + * This is the threshold cache occupancy in bytes at which we will consider an
> * RMID available for re-allocation.
> */
> -unsigned int resctrl_cqm_threshold;
> +unsigned int resctrl_rmid_realloc_threshold;
>
> #define CF(cf) ((unsigned long)(1048576 * (cf) + 0.5))
>
> @@ -223,14 +223,13 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
> */
> void __check_limbo(struct rdt_domain *d, bool force_free)
> {
> + struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> + struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
> struct rmid_entry *entry;
> - struct rdt_resource *r;
> u32 crmid = 1, nrmid;
> bool rmid_dirty;
> u64 val = 0;
>
> - r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> -
> /*
> * Skip RMID 0 and start from RMID 1 and check all the RMIDs that
> * are marked as busy for occupancy < threshold. If the occupancy
> @@ -245,10 +244,12 @@ void __check_limbo(struct rdt_domain *d, bool force_free)
> entry = __rmid_entry(nrmid);
>
> if (resctrl_arch_rmid_read(r, d, entry->rmid,
> - QOS_L3_OCCUP_EVENT_ID, &val))
> + QOS_L3_OCCUP_EVENT_ID, &val)) {
> rmid_dirty = true;
> - else
> - rmid_dirty = (val >= resctrl_cqm_threshold);
> + } else {
> + val *= hw_res->mon_scale;
> + rmid_dirty = (val >= resctrl_rmid_realloc_threshold);
> + }
>
> if (force_free || !rmid_dirty) {
> clear_bit(entry->rmid, d->rmid_busy_llc);
> @@ -289,13 +290,12 @@ int alloc_rmid(void)
>
> static void add_rmid_to_limbo(struct rmid_entry *entry)
> {
> - struct rdt_resource *r;
> + struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> + struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
> struct rdt_domain *d;
> int cpu, err;
> u64 val = 0;
>
> - r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
> -
> entry->busy = 0;
> cpu = get_cpu();
> list_for_each_entry(d, &r->domains, list) {
> @@ -303,7 +303,8 @@ static void add_rmid_to_limbo(struct rmid_entry *entry)
> err = resctrl_arch_rmid_read(r, d, entry->rmid,
> QOS_L3_OCCUP_EVENT_ID,
> &val);
> - if (err || val <= resctrl_cqm_threshold)
> + val *= hw_res->mon_scale;
> + if (err || val <= resctrl_rmid_realloc_threshold)
> continue;
> }
>
> @@ -744,6 +745,7 @@ int rdt_get_mon_l3_config(struct rdt_resource *r)
> unsigned int mbm_offset = boot_cpu_data.x86_cache_mbm_width_offset;
> struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
> unsigned int cl_size = boot_cpu_data.x86_cache_size;
> + unsigned int threshold;
> int ret;
>
> hw_res->mon_scale = boot_cpu_data.x86_cache_occ_scale;
> @@ -762,10 +764,15 @@ int rdt_get_mon_l3_config(struct rdt_resource *r)
> *
> * For a 35MB LLC and 56 RMIDs, this is ~1.8% of the LLC.
> */
> - resctrl_cqm_threshold = cl_size * 1024 / r->num_rmid;
> + threshold = cl_size * 1024 / r->num_rmid;
>
> - /* h/w works in units of "boot_cpu_data.x86_cache_occ_scale" */
Could you please keep this comment? This comment is still helpful and
meaningful in the context.
> - resctrl_cqm_threshold /= hw_res->mon_scale;
> + /*
> + * Because num_rmid may not be a power of two, round the value
> + * to the nearest multiple of hw_res->mon_scale so it matches a
> + * value the hardware will measure. mon_scale may not be a power of 2.
> + */
> + threshold /= hw_res->mon_scale;
> + resctrl_rmid_realloc_threshold = threshold * hw_res->mon_scale;
>
> ret = dom_data_init(r);
> if (ret)
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index f494ca6b8bdd..7c35561e5216 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1030,10 +1030,7 @@ static int rdt_delay_linear_show(struct kernfs_open_file *of,
> static int max_threshold_occ_show(struct kernfs_open_file *of,
> struct seq_file *seq, void *v)
> {
> - struct rdt_resource *r = of->kn->parent->priv;
> - struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
> -
> - seq_printf(seq, "%u\n", resctrl_cqm_threshold * hw_res->mon_scale);
> + seq_printf(seq, "%u\n", resctrl_rmid_realloc_threshold);
>
> return 0;
> }
> @@ -1055,7 +1052,6 @@ static int rdt_thread_throttle_mode_show(struct kernfs_open_file *of,
> static ssize_t max_threshold_occ_write(struct kernfs_open_file *of,
> char *buf, size_t nbytes, loff_t off)
> {
> - struct rdt_hw_resource *hw_res;
> unsigned int bytes;
> int ret;
>
> @@ -1066,8 +1062,7 @@ static ssize_t max_threshold_occ_write(struct kernfs_open_file *of,
> if (bytes > (boot_cpu_data.x86_cache_size * 1024))
> return -EINVAL;
>
> - hw_res = resctrl_to_arch_res(of->kn->parent->priv);
> - resctrl_cqm_threshold = bytes / hw_res->mon_scale;
> + resctrl_rmid_realloc_threshold = bytes;
Shouldn't bytes be multiples of hw_res->mon_scale? If user inputs non-multiples
value, resctrl_rmid_realloc_threshold will keep the value in the kernel. Is that
right?
But if you convert the input into multiples, user may see a different value when
read it.
Does this argument override the reason why this patch is needed?
>
> return nbytes;
> }
> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index cc6311a40582..fc4805901f1a 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -250,4 +250,6 @@ int resctrl_arch_rmid_read(struct rdt_resource *r, struct rdt_domain *d,
> void resctrl_arch_reset_rmid(struct rdt_resource *r, struct rdt_domain *d,
> u32 rmid, enum resctrl_event_id eventid);
>
> +extern unsigned int resctrl_rmid_realloc_threshold;
> +
> #endif /* _RESCTRL_H */
> --
> 2.30.2
>
Thanks.
-Fenghua
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