From: Konstantin Ananyev <konstantin.ananyev@huawei.com>
To: "dev@dpdk.org" <dev@dpdk.org>
Cc: "honnappa.nagarahalli@arm.com" <honnappa.nagarahalli@arm.com>,
Jerin Jacob <jerinj@marvell.com>,
Wathsala Vithanage <wathsala.vithanage@arm.com>,
"drc@linux.ibm.com" <drc@linux.ibm.com>
Subject: rte_ring move head question for machines with relaxed MO (arm/ppc)
Date: Tue, 8 Oct 2024 12:58:11 +0000 [thread overview]
Message-ID: <8139916ad4814629b8804525bd785d58@huawei.com> (raw)
Hi lads,
Looking at rte_ring move_head functions I noticed that all of them
use slightly different approach to guarantee desired order of memory accesses:
1. rte_ring_generic_pvt.h:
=====================
pseudo-c-code // related armv8 instructions
-------------------- --------------------------------------
head.load() // ldr [head]
rte_smp_rmb() // dmb ishld
opposite_tail.load() // ldr [opposite_tail]
...
rte_atomic32_cmpset(head, ...) // ldrex[head];... stlex[head]
2. rte_ring_c11_pvt.h
=====================
pseudo-c-code // related armv8 instructions
-------------------- --------------------------------------
head.atomic_load(relaxed) // ldr[head]
atomic_thread_fence(acquire) // dmb ish
opposite_tail.atomic_load(acquire) // lda[opposite_tail]
...
head.atomic_cas(..., relaxed) // ldrex[haed]; ... strex[head]
3. rte_ring_hts_elem_pvt.h
==========================
pseudo-c-code // related armv8 instructions
-------------------- --------------------------------------
head.atomic_load(acquire) // lda [head]
opposite_tail.load() // ldr [opposite_tail]
...
head.atomic_cas(..., acquire) // ldaex[head]; ... strex[head]
The questions that arose from these observations:
a) are all 3 approaches equivalent in terms of functionality?
b) if yes, is there any difference in terms of performance between:
"ldr; dmb; ldr;" vs "lda; ldr;"
?
c) Comapring at 1) and 2) above, combination of
ldr [head]; dmb; lda [opposite_tail]:
looks like an overkill to me. Wouldn't just:
ldr [head]; dmb; ldr[opposite_tail];
be sufficient here?
I.E.- for reading tail value - we don't need to use load(acquire).
Or probably I do miss something obvious here?
Thanks
Konstantin
For convenience, I created a godbot page with all these variants:
https://godbolt.org/z/Yjj73b8xa
#1 - __rte_ring_headtail_move_head()
#2 - __rte_ring_headtail_move_head_c11_v1
#3 - __rte_ring_headtail_move_head_c11_v2
#2 with c) - __rte_ring_headtail_move_head_c11_v3
next reply other threads:[~2024-10-08 12:58 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-10-08 12:58 Konstantin Ananyev [this message]
2024-10-08 15:09 ` Wathsala Wathawana Vithanage
2024-10-08 15:12 ` Wathsala Wathawana Vithanage
2024-10-08 15:45 ` Konstantin Ananyev
2024-10-08 15:56 ` Konstantin Ananyev
2024-10-09 17:27 ` Wathsala Wathawana Vithanage
2024-10-10 16:54 ` Konstantin Ananyev
2024-10-11 0:11 ` Wathsala Wathawana Vithanage
2024-10-11 14:08 ` Konstantin Ananyev
2024-10-11 15:48 ` Wathsala Wathawana Vithanage
2024-10-15 15:11 ` Konstantin Ananyev
2024-10-09 1:41 ` Wathsala Wathawana Vithanage
2024-10-09 2:22 ` Wathsala Wathawana Vithanage
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=8139916ad4814629b8804525bd785d58@huawei.com \
--to=konstantin.ananyev@huawei.com \
--cc=dev@dpdk.org \
--cc=drc@linux.ibm.com \
--cc=honnappa.nagarahalli@arm.com \
--cc=jerinj@marvell.com \
--cc=wathsala.vithanage@arm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).