Re: [PATCH v3] tracing: add 'accounted' entry into output of allocation tracepoints

Vasily Averin
2022-05-19T16:29:46Z

On 5/19/22 17:03, Steven Rostedt wrote:
> On Thu, 19 May 2022 14:35:46 +0300
> Vasily Averin <vvs@openvz.org> wrote:
>
>>>> @@ -33,42 +35,46 @@ DECLARE_EVENT_CLASS(kmem_alloc,
>>>> __entry->bytes_req = bytes_req;
>>>> __entry->bytes_alloc = bytes_alloc;
>>>> __entry->gfp_flags = (__force unsigned long)gfp_flags;
>>>> + __entry->accounted = (gfp_flags & __GFP_ACCOUNT) ||
>>>> + (s && s->flags & SLAB_ACCOUNT);
>>>
>>> Now you could make this even faster in the fast path and save just the
>>> s->flags.
>>>
>>> __entry->sflags = s ? s->flags : 0;
>>>
>>>> ),
>>>>
>>>> - TP_printk("call_site=%pS ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s",
>>>> + TP_printk("call_site=%pS ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s accounted=%s",
>>>> (void *)__entry->call_site,
>>>> __entry->ptr,
>>>> __entry->bytes_req,
>>>> __entry->bytes_alloc,
>>>> - show_gfp_flags(__entry->gfp_flags))
>>>> + show_gfp_flags(__entry->gfp_flags),
>>>> + __entry->accounted ? "true" : "false")
>>>
>>> And then have: "accounted=%s":
>>>
>>> (__entry->gfp_flags & __GFP_ACCOUNT) ||
>>> (__entry->sflags & SLAB_ACCOUNT) ? "true" : "false"
>>
>> Unfortunately this returns back sparse warnings about bitwise gfp_t and slab_flags_t casts.
>> Could you please explain why your variant is faster?
>
> Micro-optimization, grant you, but it is faster because it moves some of
> the logic into the slow path (the read side), and takes it out of the fast
> path (the write side).
>
> The idea of tracing is to squeeze out every cycle we can to keep the
> tracing overhead down.
>
> But it's really up to you if you need that. I'm not going to let this be a
> blocker. This is more of an FYI than anything else.

Frankly speaking I vote for performance with both hands.
However I'm still would like to avoid new sparse warnings.
Christoph Hellwig just recently taught me, "never add '__force' before
thinking hard about them", but in this case I would need to use it three times.

I found that bitwise typecasts can be avoided by using translation unions.

What do you think about following trick?

diff --git a/mm/slab.h b/mm/slab.h
index 95eb34174c1b..f676612ca40f 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -882,4 +882,14 @@ void __check_heap_object(const void *ptr, unsigned long n,
}
#endif

+union gfp_flags_u {
+ unsigned long ulong;
+ gfp_t flags;
+};
+
+union slab_flags_u {
+ unsigned int uint;
+ slab_flags_t sflags;
+};
+
#endif /* MM_SLAB_H */

diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index 71c141804222..91632a61e16d 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -13,18 +13,20 @@ DECLARE_EVENT_CLASS(kmem_alloc,

TP_PROTO(unsigned long call_site,
const void *ptr,
+ struct kmem_cache *s,
size_t bytes_req,
size_t bytes_alloc,
gfp_t gfp_flags),

- TP_ARGS(call_site, ptr, bytes_req, bytes_alloc, gfp_flags),
+ TP_ARGS(call_site, ptr, s, bytes_req, bytes_alloc, gfp_flags),

TP_STRUCT__entry(
__field( unsigned long, call_site )
__field( const void *, ptr )
__field( size_t, bytes_req )
__field( size_t, bytes_alloc )
- __field( unsigned long, gfp_flags )
+ __field_struct( union gfp_flags_u, gfp )
+ __field_struct( union slab_flags_u, s )
),

TP_fast_assign(
@@ -32,51 +34,57 @@ DECLARE_EVENT_CLASS(kmem_alloc,
__entry->ptr = ptr;
__entry->bytes_req = bytes_req;
__entry->bytes_alloc = bytes_alloc;
- __entry->gfp_flags = (__force unsigned long)gfp_flags;
+ __entry->gfp.flags = gfp_flags;
+ __entry->s.sflags = s ? s->flags : 0;
),

- TP_printk("call_site=%pS ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s",
+ TP_printk("call_site=%pS ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s accounted=%s",
(void *)__entry->call_site,
__entry->ptr,
__entry->bytes_req,
__entry->bytes_alloc,
- show_gfp_flags(__entry->gfp_flags))
+ show_gfp_flags(__entry->gfp.ulong),
+ ((__entry->gfp.flags & __GFP_ACCOUNT) ||
+ (__entry->s.sflags & SLAB_ACCOUNT)) ? "true" : "false")
);

Thank you,
Vasily Averin