[dpdk-dev,v3] mbuf: cleanup rte_pktmbuf_lastseg(), fix atomic usage
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Hi,
On Wed, Nov 15, 2017 at 12:46:15PM +0000, Hanoch Haim (hhaim) wrote:
> +Oliver,
> Ilia, I assume there is a reason why Oliver did that, I just consolidate the code.
> He didn't want to *write* the same value to mbuf field.
> In the common case that pointer was already cleared by the driver, it is better to just read it. From cache synchronization perspective it will run faster.
>
> Thanks,
> Hanoh
>
>
> -----Original Message-----
> From: Ilya Matveychikov [mailto:matvejchikov@gmail.com]
> Sent: Wednesday, November 15, 2017 1:14 PM
> To: Hanoch Haim (hhaim)
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v3] mbuf: cleanup rte_pktmbuf_lastseg(), fix atomic usage
>
>
> > On Nov 15, 2017, at 1:14 PM, Hanoh Haim <hhaim@cisco.com> wrote:
> >
I think the patch should be renamed in something like:
mbuf: fix mbuf free performance with non atomic refcnt
A description of the problem in the commit log would also be welcome.
It looks it is a regression introduced by commit 8f094a9ac5d7.
In that case, we should also have:
Fixes: 8f094a9ac5d7 ("mbuf: set mbuf fields while in pool")
> > Signed-off-by: Hanoh Haim <hhaim@cisco.com>
> > ---
> > lib/librte_mbuf/rte_mbuf.h | 27 +++++++++++++--------------
> > 1 file changed, 13 insertions(+), 14 deletions(-)
> >
> > diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> > index 7e326bb..ab110f8 100644
> > --- a/lib/librte_mbuf/rte_mbuf.h
> > +++ b/lib/librte_mbuf/rte_mbuf.h
> > @@ -1159,6 +1159,15 @@ static inline void rte_pktmbuf_reset(struct rte_mbuf *m)
> > __rte_mbuf_sanity_check(m, 1);
> > }
> >
> > +
> > +static __rte_always_inline void rte_pktmbuf_reset_before_free(struct
> > +rte_mbuf *m) {
> > + if (m->next != NULL) {
> > + m->next = NULL;
> > + m->nb_segs = 1;
> > + }
> > +}
> > +
>
> Probably it will be more clean to add something __te_pktmbuf_reset_nb_segs() without check for (m->next != NULL) and use it everywhere in mbuf’s the code, not only in
> rte_pktmbuf_prefree_seg() function. And I think it will be better to have separate patch for that.
I'm not convinced that:
__rte_pktmbuf_reset_nb_segs(m);
is clearer than:
m->next = NULL;
m->nb_segs = 1;
Anyway, I agree this should not be part of this patch. We should only keep
the fix.
>
> > /**
> > * Allocate a new mbuf from a mempool.
> > *
> > @@ -1323,8 +1332,7 @@ static inline void rte_pktmbuf_detach(struct rte_mbuf *m)
> > m->ol_flags = 0;
> >
> > if (rte_mbuf_refcnt_update(md, -1) == 0) {
> > - md->next = NULL;
> > - md->nb_segs = 1;
>
> Using rte_pktmbuf_reset_before_free() here adds unnecessary check for m->next in that path.
Yes, agree with Ilya.
>
> > + rte_pktmbuf_reset_before_free(md);
> > rte_mbuf_refcnt_set(md, 1);
> > rte_mbuf_raw_free(md);
> > }
> > @@ -1354,25 +1362,16 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > if (RTE_MBUF_INDIRECT(m))
> > rte_pktmbuf_detach(m);
> >
> > - if (m->next != NULL) {
> > - m->next = NULL;
> > - m->nb_segs = 1;
> > - }
> > -
> > + rte_pktmbuf_reset_before_free(m);
> > return m;
> >
> > - } else if (rte_atomic16_add_return(&m->refcnt_atomic, -1) == 0) {
> > -
> > + } else if (rte_mbuf_refcnt_update(m, -1) == 0) {
I agree with Konstantin's comment done in another thread [1]:
'''
That would cause extra read; cmp (and possible slowdown) for atomic refcnt.
If that really need to be fixed - probably we need to introduce a new function
that would do update without trying to read refctn first - rte_mbuf_refcnt_update_blind() or so.
'''
However I'm not sure a new function is really needed: the name is not ideal,
and it would only be used once. What about the patch below?
==============================
[1] http://dpdk.org/dev/patchwork/patch/31378/
Regards,
Olivier
Comments
Hi Olivier,
> I agree with Konstantin's comment done in another thread [1]:
>
> '''
> That would cause extra read; cmp (and possible slowdown) for atomic refcnt.
> If that really need to be fixed - probably we need to introduce a new function
> that would do update without trying to read refctn first - rte_mbuf_refcnt_update_blind() or so.
> '''
>
> However I'm not sure a new function is really needed: the name is not ideal,
That was just the first one from top of my head :)
If the issue in naming only - I suppose we can find something better.
Personally I like Ilya's one: __rte_mbuf_refcnt_update().
> and it would only be used once. What about the patch below?
It would do the job too, but looks a bit clumsy to me.
Konstantin
>
> ==============================
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -1361,8 +1361,18 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>
> return m;
>
> - } else if (rte_atomic16_add_return(&m->refcnt_atomic, -1) == 0) {
> + } else {
>
> + /* We don't use rte_mbuf_refcnt_update() because we already
> + * tested that refcnt != 1.
> + */
> +#ifdef RTE_MBUF_REFCNT_ATOMIC
> + ret = rte_atomic16_add_return(&m->refcnt_atomic, -1);
> +#else
> + ret = --m->refcnt;
> +#endif
> + if (ret != 0)
> + return NULL;
>
> if (RTE_MBUF_INDIRECT(m))
> rte_pktmbuf_detach(m);
> @@ -1375,7 +1385,6 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>
> return m;
> }
> - return NULL;
> }
>
> /* deprecated, replaced by rte_pktmbuf_prefree_seg() */
> ==============================
>
> [1] http://dpdk.org/dev/patchwork/patch/31378/
>
>
> Regards,
> Olivier
==============================
@@ -1361,8 +1361,18 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
return m;
- } else if (rte_atomic16_add_return(&m->refcnt_atomic, -1) == 0) {
+ } else {
+ /* We don't use rte_mbuf_refcnt_update() because we already
+ * tested that refcnt != 1.
+ */
+#ifdef RTE_MBUF_REFCNT_ATOMIC
+ ret = rte_atomic16_add_return(&m->refcnt_atomic, -1);
+#else
+ ret = --m->refcnt;
+#endif
+ if (ret != 0)
+ return NULL;
if (RTE_MBUF_INDIRECT(m))
rte_pktmbuf_detach(m);
@@ -1375,7 +1385,6 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
return m;
}
- return NULL;
}
/* deprecated, replaced by rte_pktmbuf_prefree_seg() */