Hi Jerin
On 10/20/2017 1:43 PM, Jerin Jacob Wrote:
-----Original Message-----
[...]
dependant on each other.
Thus a memory barrier is neccessary.
Yes. The barrier is necessary.
In fact, upstream freebsd fixed this issue for arm64. DPDK ring
implementation is derived from freebsd's buf_ring.h.
https://github.com/freebsd/freebsd/blob/master/sys/sys/buf_ring.h#L166
I think, the only outstanding issue is, how to reduce the performance
impact for arm64. I believe using accurate/release semantics instead
of rte_smp_rmb() will reduce the performance overhead like similar ring
implementations below,
freebsd: https://github.com/freebsd/freebsd/blob/master/sys/sys/buf_ring.h#L166
odp:
https://github.com/Linaro/odp/blob/master/platform/linux-generic/pktio/ring.c
Jia,
1) Can you verify the use of accurate/release semantics fixes the problem in
your
platform? like use of atomic_load_acq* in the reference code.
2) If so, What is the overhead between accurate/release and plane smp_smb()
barriers. Based on that we need decide what path to take.
I've tested 3 cases. The new 3rd case is to use the load_acquire
barrier (half barrier) you mentioned
at above link.
The patch seems like:
@@ -408,8 +466,8 @@ __rte_ring_move_prod_head(struct rte_ring *r, int is_sp,
/* Reset n to the initial burst count */
n = max;
- *old_head = r->prod.head;
- const uint32_t cons_tail = r->cons.tail;
+ *old_head = atomic_load_acq_32(&r->prod.head);
+ const uint32_t cons_tail =
atomic_load_acq_32(&r->cons.tail);
@@ -516,14 +576,15 @@ __rte_ring_move_cons_head(struct rte_ring *r, int is_s
/* Restore n as it may change every loop */
n = max;
- *old_head = r->cons.head;
- const uint32_t prod_tail = r->prod.tail;
+ *old_head = atomic_load_acq_32(&r->cons.head);
+ const uint32_t prod_tail = atomic_load_acq_32(&r->prod.tail)
/* The subtraction is done between two unsigned 32bits
value
* (the result is always modulo 32 bits even if we have
* cons_head > prod_tail). So 'entries' is always between 0
* and size(ring)-1. */
The half barrier patch passed the fuctional test.
As for the performance comparision on *arm64*(the debug patch is at
http://dpdk.org/ml/archives/dev/2017-October/079012.html), please see
the test results
below:
[case 1] old codes, no barrier
============================================
Performance counter stats for './test --no-huge -l 1-10':
689275.001200 task-clock (msec) # 9.771 CPUs utilized
6223 context-switches # 0.009 K/sec
10 cpu-migrations # 0.000 K/sec
653 page-faults # 0.001 K/sec
1721190914583 cycles # 2.497 GHz
3363238266430 instructions # 1.95 insn per
cycle
<not supported> branches
27804740 branch-misses # 0.00% of all
branches
70.540618825 seconds time elapsed
[case 2] full barrier with rte_smp_rmb()
============================================
Performance counter stats for './test --no-huge -l 1-10':
582557.895850 task-clock (msec) # 9.752 CPUs utilized
5242 context-switches # 0.009 K/sec
10 cpu-migrations # 0.000 K/sec
665 page-faults # 0.001 K/sec
1454360730055 cycles # 2.497 GHz
587197839907 instructions # 0.40 insn per
cycle
<not supported> branches
27799687 branch-misses # 0.00% of all
branches
59.735582356 seconds time elapse
[case 1] half barrier with load_acquire
============================================
Performance counter stats for './test --no-huge -l 1-10':
660758.877050 task-clock (msec) # 9.764 CPUs utilized
5982 context-switches # 0.009 K/sec
11 cpu-migrations # 0.000 K/sec
657 page-faults # 0.001 K/sec
1649875318044 cycles # 2.497 GHz
591583257765 instructions # 0.36 insn per
cycle
<not supported> branches
27994903 branch-misses # 0.00% of all
branches
67.672855107 seconds time elapsed
Please see the context-switches in the perf results
test result sorted by time is:
full barrier < half barrier < no barrier
AFAICT, in this case ,the cpu reordering will add the possibility for
context switching and
increase the running time.
Any ideas?
Cheers,
Jia
Note:
This issue wont come in all the arm64 implementation. it comes on arm64
implementation with OOO(out of order) implementations.
Cheers,
Jia
Konstantin
. In another
mail of this thread, we've made a simple test based on this and captured
some information and I pasted there.(I pasted the patch there :-))
Are you talking about that one:
http://dpdk.org/dev/patchwork/patch/30405/
?
It still uses test/test/test_mbuf.c...,
but anyway I don't really understand how mbuf_autotest supposed
to work with these changes:
@@ -730,7 +739,7 @@ test_refcnt_iter(unsigned int lcore, unsigned int iter,
rte_ring_enqueue(refcnt_mbuf_ring, m);
}
}
- rte_pktmbuf_free(m);
+ // rte_pktmbuf_free(m);
}
@@ -741,6 +750,12 @@ test_refcnt_iter(unsigned int lcore, unsigned int iter,
while (!rte_ring_empty(refcnt_mbuf_ring))
;
+ if (NULL != m) {
+ if (1 != rte_mbuf_refcnt_read(m))
+ printf("m ref is %u\n", rte_mbuf_refcnt_read(m));
+ rte_pktmbuf_free(m);
+ }
+
/* check that all mbufs are back into mempool by now */
for (wn = 0; wn != REFCNT_MAX_TIMEOUT; wn++) {
if ((i = rte_mempool_avail_count(refcnt_pool)) == n) {
That means all your mbufs (except the last one) will still be allocated.
So the test would fail - as it should, I think.
And
it seems that Juhamatti & Jacod found some reverting action several
months ago.
Didn't get that one either.
Konstantin
--
Cheers,
Jia
