netdev - Re: [PATCH net-next 01/14] net_sched: sch_fq: implement lockless fq

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Message-ID: <CANn89i+iNKvCv+RPtCa4KOY9DCEQJfGP9xHSedFUbWZHt2DSFw@mail.gmail.com>
Date: Wed, 17 Apr 2024 10:45:09 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Simon Horman <horms@...nel.org>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Jamal Hadi Salim <jhs@...atatu.com>, 
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com
Subject: Re: [PATCH net-next 01/14] net_sched: sch_fq: implement lockless fq_dump()

On Tue, Apr 16, 2024 at 8:33 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Apr 16, 2024 at 8:19 PM Simon Horman <horms@...nel.org> wrote:
> >
> > On Mon, Apr 15, 2024 at 01:20:41PM +0000, Eric Dumazet wrote:
> > > Instead of relying on RTNL, fq_dump() can use READ_ONCE()
> > > annotations, paired with WRITE_ONCE() in fq_change()
> > >
> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > ---
> > >  net/sched/sch_fq.c | 96 +++++++++++++++++++++++++++++-----------------
> > >  1 file changed, 60 insertions(+), 36 deletions(-)
> > >
> > > diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
> > > index cdf23ff16f40bf244bb822e76016fde44e0c439b..934c220b3f4336dc2f70af74d7758218492b675d 100644
> > > --- a/net/sched/sch_fq.c
> > > +++ b/net/sched/sch_fq.c
> > > @@ -888,7 +888,7 @@ static int fq_resize(struct Qdisc *sch, u32 log)
> > >               fq_rehash(q, old_fq_root, q->fq_trees_log, array, log);
> > >
> > >       q->fq_root = array;
> > > -     q->fq_trees_log = log;
> > > +     WRITE_ONCE(q->fq_trees_log, log);
> > >
> > >       sch_tree_unlock(sch);
> > >
> > > @@ -931,7 +931,7 @@ static void fq_prio2band_compress_crumb(const u8 *in, u8 *out)
> > >
> > >       memset(out, 0, num_elems / 4);
> > >       for (i = 0; i < num_elems; i++)
> > > -             out[i / 4] |= in[i] << (2 * (i & 0x3));
> > > +             out[i / 4] |= READ_ONCE(in[i]) << (2 * (i & 0x3));
> > >  }
> > >
> >
> > Hi Eric,
> >
> > I am a little unsure about the handling of q->prio2band in this patch.
> >
> > It seems to me that fq_prio2band_compress_crumb() is used to
> > to store values in q->prio2band, and is called (indirectly)
> > from fq_change() (and directly from fq_init()).
> >
> > While fq_prio2band_decompress_crumb() is used to read values
> > from q->prio2band, and is called from fq_dump().
> >
> > So I am wondering if should use WRITE_ONCE() when storing elements
> > of out. And fq_prio2band_decompress_crumb should use READ_ONCE when
> > reading elements of in.
>
> Yeah, you are probably right, I recall being a bit lazy on this part,
> thanks !

I will squash in V2 this part :

diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index 934c220b3f4336dc2f70af74d7758218492b675d..238974725679327b0a0d483c011e15fc94ab0878
100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -106,6 +106,8 @@ struct fq_perband_flows {
        int                 quantum; /* based on band nr : 576KB, 192KB, 64KB */
 };

+#define FQ_PRIO2BAND_CRUMB_SIZE ((TC_PRIO_MAX + 1) >> 2)
+
 struct fq_sched_data {
 /* Read mostly cache line */

@@ -122,7 +124,7 @@ struct fq_sched_data {
        u8              rate_enable;
        u8              fq_trees_log;
        u8              horizon_drop;
-       u8              prio2band[(TC_PRIO_MAX + 1) >> 2];
+       u8              prio2band[FQ_PRIO2BAND_CRUMB_SIZE];
        u32             timer_slack; /* hrtimer slack in ns */

 /* Read/Write fields. */
@@ -159,7 +161,7 @@ struct fq_sched_data {
 /* return the i-th 2-bit value ("crumb") */
 static u8 fq_prio2band(const u8 *prio2band, unsigned int prio)
 {
-       return (prio2band[prio / 4] >> (2 * (prio & 0x3))) & 0x3;
+       return (READ_ONCE(prio2band[prio / 4]) >> (2 * (prio & 0x3))) & 0x3;
 }

 /*
@@ -927,11 +929,15 @@ static const struct nla_policy
fq_policy[TCA_FQ_MAX + 1] = {
 static void fq_prio2band_compress_crumb(const u8 *in, u8 *out)
 {
        const int num_elems = TC_PRIO_MAX + 1;
+       u8 tmp[FQ_PRIO2BAND_CRUMB_SIZE];
        int i;

-       memset(out, 0, num_elems / 4);
+       memset(tmp, 0, sizeof(tmp));
        for (i = 0; i < num_elems; i++)
-               out[i / 4] |= READ_ONCE(in[i]) << (2 * (i & 0x3));
+               tmp[i / 4] |= in[i] << (2 * (i & 0x3));
+
+       for (i = 0; i < FQ_PRIO2BAND_CRUMB_SIZE; i++)
+               WRITE_ONCE(out[i], tmp[i]);
 }

 static void fq_prio2band_decompress_crumb(const u8 *in, u8 *out)