On Fri, Apr 12, 2024 at 4:38 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 05/04/2024 11:27, Barry Song wrote:
> > From: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > Profiling a system blindly with mTHP has become challenging due to the
> > lack of visibility into its operations. Presenting the success rate of
> > mTHP allocations appears to be pressing need.
> >
> > Recently, I've been experiencing significant difficulty debugging
> > performance improvements and regressions without these figures.
> > It's crucial for us to understand the true effectiveness of mTHP in
> > real-world scenarios, especially in systems with fragmented memory.
> >
> > This patch sets up the framework for per-order mTHP counters, starting
> > with the introduction of anon_alloc and anon_alloc_fallback counters.
> > Incorporating additional counters should now be straightforward as well.
> >
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > ---
> > include/linux/huge_mm.h | 19 ++++++++++++++++
> > mm/huge_memory.c | 48 +++++++++++++++++++++++++++++++++++++++++
> > mm/memory.c | 2 ++
> > 3 files changed, 69 insertions(+)
> >
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index e896ca4760f6..c5d33017a4dd 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -264,6 +264,25 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
> > enforce_sysfs, orders);
> > }
> >
> > +enum mthp_stat_item {
> > + MTHP_STAT_ANON_ALLOC,
> > + MTHP_STAT_ANON_ALLOC_FALLBACK,
> > + __MTHP_STAT_COUNT
> > +};
> > +
> > +struct mthp_stat {
> > + unsigned long stats[PMD_ORDER + 1][__MTHP_STAT_COUNT];
>
> I saw a fix for this allocation dynamically due to powerpc PMD_ORDER not being
> constant. I wonder if ilog2(MAX_PTRS_PER_PTE) would help here?
>
It's a possibility. However, since we've passed all the build tests
using dynamic
allocation, it might not be worth the effort to attempt static
allocation again. Who
knows what will happen next :-)
> > +};
> > +
> > +DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
> > +
> > +static inline void count_mthp_stat(int order, enum mthp_stat_item item)
>
> I thought we were going to call this always counting up type of stat and event?
> "count_mthp_event"? But I'm happy with it as is, personally.
>
> > +{
> > + if (unlikely(order > PMD_ORDER))
> > + return;
>
> I'm wondering if it also makes sense to ignore order == 0? Although I guess if
> called for order-0 its safe since the storage exists and sum_mthp_stat() is
> never be called for 0. Ignore this comment :)
Agreed. I'd like to change it to ignore oder 0;
>
> > + this_cpu_inc(mthp_stats.stats[order][item]);
> > +}
> > +
> > #define transparent_hugepage_use_zero_page() \
> > (transparent_hugepage_flags & \
> > (1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG))
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 9d4b2fbf6872..5b875f0fc923 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -526,6 +526,46 @@ static const struct kobj_type thpsize_ktype = {
> > .sysfs_ops = &kobj_sysfs_ops,
> > };
> >
> > +DEFINE_PER_CPU(struct mthp_stat, mthp_stats) = {{{0}}};
> > +
> > +static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)
> > +{
> > + unsigned long sum = 0;
> > + int cpu;
> > +
> > + for_each_online_cpu(cpu) {
>
> What happens if a cpu that was online and collected a bunch of stats gets
> offlined? The user will see stats get smaller?
>
> Perhaps this should be for_each_possible_cpu()? Although I'm not sure what
> happens to percpu data when a cpu goes offline? Is the data preserved? Or wiped,
> or unmapped? dunno. Might we need to rescue stats into a global counter at
> offline-time?
Good catch. I see /proc/vmstat is always using the for_each_online_cpu() but it
doesn't have the issue, but mTHP counters do have the problem.
* step 1: cat the current thp_swpout value before running a test
program which does
swpout;
/ # cat /proc/vmstat | grep thp_swpout
thp_swpout 0
/ # cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/stats/anon_swpout
0
* step 2: run the test program on cpu2;
/ # taskset -c 2 /home/barry/develop/linux/swpcache-2m
* step 3: cat the current thp_swpout value after running a test
program which does
swpout;
/ # cat /proc/vmstat | grep thp_swpout
thp_swpout 98
/ # cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/stats/anon_swpout
98
*step 4: offline cpu2 and read thp_swpout;
/ # echo 0 > /sys/devices/system/cpu/cpu2/online
[ 339.058661] psci: CPU2 killed (polled 0 ms)
/ # cat /proc/vmstat | grep thp_swpout
thp_swpout 98
/ # cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/stats/anon_swpout
0
*step 5: online cpu2 and read thp_swpout
/ # echo 1 > /sys/devices/system/cpu/cpu2/online
[ 791.642058] CPU2: Booted secondary processor 0x0000000002 [0x000f0510]
/ # cat /proc/vmstat | grep thp_swpout
thp_swpout 98
/ # cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/stats/anon_swpout
98
As you can see, in step 4, /proc/vmstat is all good but mTHP counters become
zero.
The reason is /proc/vmstat will fold the offline cpu to an online cpu
but mthp counters lack
it:
/*
* Fold the foreign cpu events into our own.
*
* This is adding to the events on one processor
* but keeps the global counts constant.
*/
void vm_events_fold_cpu(int cpu)
{
struct vm_event_state *fold_state = &per_cpu(vm_event_states, cpu);
int i;
for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
count_vm_events(i, fold_state->event[i]);
fold_state->event[i] = 0;
}
}
static int page_alloc_cpu_dead(unsigned int cpu)
{
...
/*
* Spill the event counters of the dead processor
* into the current processors event counters.
* This artificially elevates the count of the current
* processor.
*/
vm_events_fold_cpu(cpu);
...
return 0;
}
So I will do the same thing for mTHP counters - fold offline cpu
counters to online one.
>
> > + struct mthp_stat *this = &per_cpu(mthp_stats, cpu);
> > +
> > + sum += this->stats[order][item];
> > + }
> > +
> > + return sum;
> > +}
> > +
> > +#define DEFINE_MTHP_STAT_ATTR(_name, _index) \
> > +static ssize_t _name##_show(struct kobject *kobj, \
> > + struct kobj_attribute *attr, char *buf) \
> > +{ \
> > + int order = to_thpsize(kobj)->order; \
> > + \
> > + return sysfs_emit(buf, "%lu\n", sum_mthp_stat(order, _index)); \
> > +} \
> > +static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
>
> Very nice!
Right. I got duplicated copy-paste and bad small in code so I wrote this macro.
>
> > +
> > +DEFINE_MTHP_STAT_ATTR(anon_alloc, MTHP_STAT_ANON_ALLOC);
> > +DEFINE_MTHP_STAT_ATTR(anon_alloc_fallback, MTHP_STAT_ANON_ALLOC_FALLBACK);
> > +
> > +static struct attribute *stats_attrs[] = {
> > + &anon_alloc_attr.attr,
> > + &anon_alloc_fallback_attr.attr,
> > + NULL,
> > +};
> > +
> > +static struct attribute_group stats_attr_group = {
> > + .name = "stats",
> > + .attrs = stats_attrs,
> > +};
> > +
> > static struct thpsize *thpsize_create(int order, struct kobject *parent)
> > {
> > unsigned long size = (PAGE_SIZE << order) / SZ_1K;
> > @@ -549,6 +589,12 @@ static struct thpsize *thpsize_create(int order, struct kobject *parent)
> > return ERR_PTR(ret);
> > }
> >
> > + ret = sysfs_create_group(&thpsize->kobj, &stats_attr_group);
> > + if (ret) {
> > + kobject_put(&thpsize->kobj);
> > + return ERR_PTR(ret);
> > + }
> > +
> > thpsize->order = order;
> > return thpsize;
> > }
> > @@ -1050,8 +1096,10 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> > folio = vma_alloc_folio(gfp, HPAGE_PMD_ORDER, vma, haddr, true);
> > if (unlikely(!folio)) {
> > count_vm_event(THP_FAULT_FALLBACK);
> > + count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_ALLOC_FALLBACK);
>
> I think we should aim for the PMD-oder MTHP_STAT_ANON_ALLOC and
> MTHP_STAT_ANON_ALLOC_FALLBACK to match THP_FAULT_ALLOC and THP_FAULT_FALLBACK.
> Its not currently setup this way...
right. I also realized this and asked for your comments on this in another
thread.
>
>
> > return VM_FAULT_FALLBACK;
> > }
> > + count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_ALLOC);
> > return __do_huge_pmd_anonymous_page(vmf, &folio->page, gfp);
> > }
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 649e3ed94487..1723c8ddf9cb 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4374,8 +4374,10 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> > }
> > folio_throttle_swaprate(folio, gfp);
> > clear_huge_page(&folio->page, vmf->address, 1 << order);
> > + count_mthp_stat(order, MTHP_STAT_ANON_ALLOC);
> > return folio;
> > }
> > + count_mthp_stat(order, MTHP_STAT_ANON_ALLOC_FALLBACK);
>
> ...And we should follow the usage same pattern for the smaller mTHP here too.
> Which means MTHP_STAT_ANON_ALLOC_FALLBACK should be after the next: label. We
The only difference is the case
if (mem_cgroup_charge(folio, vma->vm_mm, gfp))
goto next;
but vmstat is counting this as fallback so i feel good to move after next,
if (mem_cgroup_charge(folio, vma->vm_mm, gfp)) {
folio_put(folio);
count_vm_event(THP_FAULT_FALLBACK);
count_vm_event(THP_FAULT_FALLBACK_CHARGE);
return VM_FAULT_FALLBACK;
}
> could introduce a MTHP_STAT_ANON_ALLOC_FALLBACK_CHARGE which would only trigger
> on a fallback due to charge failure, just like THP_FAULT_FALLBACK_CHARGE?
it is fine to add this THP_FAULT_FALLBACK_CHARGE though it is not that
useful for profiling buddy fragmentation.
>
> > next:
> > order = next_order(&orders, order);
> > }
>
Thanks
Barry
