Re: [PATCH v1] lib,kprobes: using try_cmpxchg_local in objpool_push
From: wuqiang.matt
Date: Mon Oct 23 2023 - 21:57:52 EST
On 2023/10/24 09:01, Masami Hiramatsu (Google) wrote:
On Mon, 23 Oct 2023 11:43:04 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
On Mon, 23 Oct 2023 19:24:52 +0800
"wuqiang.matt" <wuqiang.matt@xxxxxxxxxxxxx> wrote:
The objpool_push can only happen on local cpu node, so only the local
cpu can touch slot->tail and slot->last, which ensures the correctness
of using cmpxchg without lock prefix (using try_cmpxchg_local instead
of try_cmpxchg_acquire).
Testing with IACA found the lock version of pop/push pair costs 16.46
cycles and local-push version costs 15.63 cycles. Kretprobe throughput
is improved to 1.019 times of the lock version for x86_64 systems.
OS: Debian 10 X86_64, Linux 6.6rc6 with freelist
HW: XEON 8336C x 2, 64 cores/128 threads, DDR4 3200MT/s
1T 2T 4T 8T 16T
lock: 29909085 59865637 119692073 239750369 478005250
local: 30297523 60532376 121147338 242598499 484620355
32T 48T 64T 96T 128T
lock: 957553042 1435814086 1680872925 2043126796 2165424198
local: 968526317 1454991286 1861053557 2059530343 2171732306
Signed-off-by: wuqiang.matt <wuqiang.matt@xxxxxxxxxxxxx>
---
lib/objpool.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/lib/objpool.c b/lib/objpool.c
index ce0087f64400..a032701beccb 100644
--- a/lib/objpool.c
+++ b/lib/objpool.c
@@ -166,7 +166,7 @@ objpool_try_add_slot(void *obj, struct objpool_head *pool, int cpu)
head = READ_ONCE(slot->head);
/* fault caught: something must be wrong */
WARN_ON_ONCE(tail - head > pool->nr_objs);
- } while (!try_cmpxchg_acquire(&slot->tail, &tail, tail + 1));
+ } while (!try_cmpxchg_local(&slot->tail, &tail, tail + 1));
/* now the tail position is reserved for the given obj */
WRITE_ONCE(slot->entries[tail & slot->mask], obj);
I'm good with the change, but I don't like how "cpu" is passed to this
function. It currently is only used in one location, which does:
rc = objpool_try_add_slot(obj, pool, raw_smp_processor_id());
Which makes this change fine. But there's nothing here to prevent someone
for some reason passing another CPU to that function.
If we are to make that change, I would be much more comfortable with
removing "int cpu" as a parameter to objpool_try_add_slot() and adding:
int cpu = raw_smp_processor_id();
Which now shows that this function *only* deals with the current CPU.
Oh indeed. It used to search all CPUs to push the object, but
I asked him to stop that because there should be enough space to
push it in the local ring. This is a remnant of that time.
Yes, good catch. Thanks for the explanation.
Wuqiang, can you make another patch to fix it?
I'm thinking of removing the inline function objpool_try_add_slot and merging
its functionality to objpool_push, like the followings:
/* reclaim an object to object pool */
int objpool_push(void *obj, struct objpool_head *pool)
{
struct objpool_slot *slot;
uint32_t head, tail;
unsigned long flags;
/* disable local irq to avoid preemption & interruption */
raw_local_irq_save(flags);
slot = pool->cpu_slots[raw_smp_processor_id()];
/* loading tail and head as a local snapshot, tail first */
tail = READ_ONCE(slot->tail);
do {
head = READ_ONCE(slot->head);
/* fault caught: something must be wrong */
WARN_ON_ONCE(tail - head > pool->nr_objs);
} while (!try_cmpxchg_local(&slot->tail, &tail, tail + 1));
/* now the tail position is reserved for the given obj */
WRITE_ONCE(slot->entries[tail & slot->mask], obj);
/* update sequence to make this obj available for pop() */
smp_store_release(&slot->last, tail + 1);
raw_local_irq_restore(flags);
return 0;
}
I'll prepare a new patch for this improvement.
Thank you,
-- Steve
Thanks for your time,
wuqiang