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From:   Kan Yan <kyan@google.com>
Date:   Wed, 13 Nov 2019 18:26:23 -0800
Message-ID: <CA+iem5sp+52tT3u+uzCgv0fvfP1WX85-U31wzQ6wNKyt34RFpQ@mail.gmail.com>
Subject: Re: [PATCH v7 1/2] mac80211: Implement Airtime-based Queue Limit (AQL)
To:     =?UTF-8?B?VG9rZSBIw7hpbGFuZC1Kw7hyZ2Vuc2Vu?= <toke@redhat.com>
Cc:     Johannes Berg <johannes@sipsolutions.net>,
        linux-wireless@vger.kernel.org,
        Make-Wifi-fast <make-wifi-fast@lists.bufferbloat.net>,
        Felix Fietkau <nbd@nbd.name>, Yibo Zhao <yiboz@codeaurora.org>,
        John Crispin <john@phrozen.org>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        Rajkumar Manoharan <rmanohar@codeaurora.org>,
        Kevin Hayes <kevinhayes@google.com>
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> Oh, right, I see. But in that case, should writing the default really
> stomp on all the per-station values? If I set the value of a station, I
> wouldn't expect it to change just because I changed the default value
> afterwards?

Will persevere the value for stations with customized queue limit in
the next version.

> > That's indeed not right. However, if a potential aql_tx_pending
> > underflow case is detected here (It should never happen), reset it to
> > 0 maybe not the best remedy anyway. I think it is better  just
> > WARN_ONCE() and skip updating aql_tx_pending all together, so the
> > retry or loop can be avoided here. What do you think?
> If we don't reset the value to zero may end up with a device that is
> unable to transmit. Better to reset it I think, even if this is never
> supposed to happen...

I mean not updating the pending airtime to prevent it from going
negative when the tx_airtime is larger than aql_tx_pending.
Will reset it to 0 in next version, which is simpler and cleaner.


On Wed, Nov 13, 2019 at 6:02 AM Toke H=C3=B8iland-J=C3=B8rgensen <toke@redh=
at.com> wrote:
>
> Kan Yan <kyan@google.com> writes:
>
> > Thanks for the review. I will pick up your new patches and give it a
> > try tomorrow.
> >
> >> Why is this setting sta and device limits to the same value?
> >
> > local->aql_txq_limit_low is not the per device limit, but the default
> > txq_limit for all STAs. Individual stations can be configured with
> > non-default value via debugfs entry
> > "netdev:interface_name_x/stations/mac_addr_x/airtime". "aql_threshold"
> > is the device limit for switching between the lower and higher per
> > station queue limit.
>
> Oh, right, I see. But in that case, should writing the default really
> stomp on all the per-station values? If I set the value of a station, I
> wouldn't expect it to change just because I changed the default value
> afterwards?
>
> >> Also, are you sure we won't risk write tearing when writing 32-bit
> >> values without locking on some architectures?
> >
> > Does mac80211 ever runs in any 16-bit architectures? Even in an
> > architecture that write to 32-bit value is not atomic, I don't think
> > there is any side-effect for queue limit get wrong transiently in rare
> > occasions. Besides, the practical value of those queue limits should
> > always fit into 16 bits.
>
> I'm not sure about the platform characteristics of all the weird tiny
> MIPS boxes that run OpenWrt; which is why I'm vary of making any
> assumptions that it is safe :)
>
> But yeah, I suppose you're right that since we're just setting the
> limit, it is not going to be a huge concern here...
>
> >> I don't think this is right; another thread could do atomic_inc()
> >> between the atomic_read() and atomic_set() here, in which case this
> >> would clobber the other value.
> >> I think to get this right the logic would need to be something like
> >> this:
> >> retry:
> >>   old =3D atomic_read(&sta->airtime[ac].aql_tx_pending);
> >>   if (warn_once(tx_airtime > old))
> >>      new =3D 0;
> >>   else
> >>      new =3D old - tx_airtime;
> >>   if (atomic_cmpxchg(&sta->airtime[ac].aql_tx_pending, old, new) !=3D =
old)
> >>      goto retry;
> >> (or use an equivalent do/while).
> >
> > That's indeed not right. However, if a potential aql_tx_pending
> > underflow case is detected here (It should never happen), reset it to
> > 0 maybe not the best remedy anyway. I think it is better  just
> > WARN_ONCE() and skip updating aql_tx_pending all together, so the
> > retry or loop can be avoided here. What do you think?
>
> If we don't reset the value to zero may end up with a device that is
> unable to transmit. Better to reset it I think, even if this is never
> supposed to happen...
>
> -Toke
>