Message-ID: <CALx6S37A9QimRinL7YAcoSh8MRn6jpJ7pMZwqGLGiTeaBoBORg@mail.gmail.com>
Date:   Thu, 1 Sep 2016 16:30:28 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>
Cc:     Brenden Blanco <bblanco@...mgrid.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Or Gerlitz <gerlitz.or@...il.com>
Subject: Re: [PATCH] net/mlx4_en: protect ring->xdp_prog with rcu_read_lock

On Thu, Sep 1, 2016 at 3:59 PM, Saeed Mahameed
<saeedm@....mellanox.co.il> wrote:
> On Wed, Aug 31, 2016 at 4:50 AM, Brenden Blanco <bblanco@...mgrid.com> wrote:
>> On Tue, Aug 30, 2016 at 12:35:58PM +0300, Saeed Mahameed wrote:
>>> On Mon, Aug 29, 2016 at 8:46 PM, Tom Herbert <tom@...bertland.com> wrote:
>>> > On Mon, Aug 29, 2016 at 8:55 AM, Brenden Blanco <bblanco@...mgrid.com> wrote:
>>> >> On Mon, Aug 29, 2016 at 05:59:26PM +0300, Tariq Toukan wrote:
>>> >>> Hi Brenden,
>>> >>>
>>> >>> The solution direction should be XDP specific that does not hurt the
>>> >>> regular flow.
>>> >> An rcu_read_lock is _already_ taken for _every_ packet. This is 1/64th of
>>>
>>> In other words "let's add  new small speed bump, we already have
>>> plenty ahead, so why not slow down now anyway".
>>>
>>> Every single new instruction hurts performance, in this case maybe you
>>> are right, maybe we won't feel any performance
>>> impact, but that doesn't mean it is ok to do this.
>> Actually, I will make a stronger assertion. Unless your .config contains
>> CONFIG_PREEMPT=y (not most distros) or something like DEBUG_ATOMIC_SLEEP
>> (to trigger PREEMPT_COUNT), the code in this patch will be a nop.
>> Therefore, adding the protections that you mention below will be
>> _slower_ than the code already proposed.
>>>
>>>
>>> >> that.
>>> >>>
>>> >>> On 26/08/2016 11:38 PM, Brenden Blanco wrote:
>>> >>> >Depending on the preempt mode, the bpf_prog stored in xdp_prog may be
>>> >>> >freed despite the use of call_rcu inside bpf_prog_put. The situation is
>>> >>> >possible when running in PREEMPT_RCU=y mode, for instance, since the rcu
>>> >>> >callback for destroying the bpf prog can run even during the bh handling
>>> >>> >in the mlx4 rx path.
>>> >>> >
>>> >>> >Several options were considered before this patch was settled on:
>>> >>> >
>>> >>> >Add a napi_synchronize loop in mlx4_xdp_set, which would occur after all
>>> >>> >of the rings are updated with the new program.
>>> >>> >This approach has the disadvantage that as the number of rings
>>> >>> >increases, the speed of udpate will slow down significantly due to
>>> >>> >napi_synchronize's msleep(1).
>>> >>> I prefer this option as it doesn't hurt the data path. A delay in a
>>> >>> control command can be tolerated.
>>> >>> >Add a new rcu_head in bpf_prog_aux, to be used by a new bpf_prog_put_bh.
>>> >>> >The action of the bpf_prog_put_bh would be to then call bpf_prog_put
>>> >>> >later. Those drivers that consume a bpf prog in a bh context (like mlx4)
>>> >>> >would then use the bpf_prog_put_bh instead when the ring is up. This has
>>> >>> >the problem of complexity, in maintaining proper refcnts and rcu lists,
>>> >>> >and would likely be harder to review. In addition, this approach to
>>> >>> >freeing must be exclusive with other frees of the bpf prog, for instance
>>> >>> >a _bh prog must not be referenced from a prog array that is consumed by
>>> >>> >a non-_bh prog.
>>> >>> >
>>> >>> >The placement of rcu_read_lock in this patch is functionally the same as
>>> >>> >putting an rcu_read_lock in napi_poll. Actually doing so could be a
>>> >>> >potentially controversial change, but would bring the implementation in
>>> >>> >line with sk_busy_loop (though of course the nature of those two paths
>>> >>> >is substantially different), and would also avoid future copy/paste
>>> >>> >problems with future supporters of XDP. Still, this patch does not take
>>> >>> >that opinionated option.
>>> >>> So you decided to add a lock for all non-XDP flows, which are 99% of
>>> >>> the cases.
>>> >>> We should avoid this.
>>> >> The whole point of rcu_read_lock architecture is to be taken in the fast
>>> >> path. There won't be a performance impact from this patch.
>>> >
>>> > +1, this is nothing at all like a spinlock and really this should be
>>> > just like any other rcu like access.
>>> >
>>> > Brenden, tracking down how the structure is freed needed a few steps,
>>> > please make sure the RCU requirements are well documented. Also, I'm
>>> > still not a fan of using xchg to set the program, seems that a lock
>>> > could be used in that path.
>>> >
>>> > Thanks,
>>> > Tom
>>>
>>> Sorry folks I am with Tariq on this, you can't just add a single
>>> instruction which is only valid/needed for 1% of the use cases
>>> to the driver's general data path, even if it was as cheap as one cpu cycle!
>> How about 0?
>>
>> $ diff mlx4_en.ko.norcu.s mlx4_en.ko.rcu.s | wc -l
>> 0
>>
>
> Well, If you put it this way, it seems OK then.
>
> Anyway I would add a friendly comment beside the rcu_read_lock that
> "this is needed to protect
> access to ring->xdp_prog".
>
>>>
>>> Let me try to suggest something:
>>> instead of taking the rcu_read_lock for the whole
>>> mlx4_en_process_rx_cq, we can minimize to XDP code path only
>>> by double checking xdp_prog (non-protected check followed by a
>>> protected check inside mlx4 XDP critical path).
>>>
>>> i.e instead of:
>>>
>>> rcu_read_lock();
>>>
>>> xdp_prog = ring->xdp_prog;
>>>
>>> //__Do lots of non-XDP related stuff__
>>>
>>> if (xdp_prog) {
>>>     //Do XDP magic ..
>>> }
>>> //__Do more of non-XDP related stuff__
>>>
>>> rcu_read_unlock();
>>>
>>>
>>> We can minimize it to XDP critical path only:
>>>
>>> //Non protected xdp_prog dereference.
>>> if (xdp_prog) {
>>>      rcu_read_lock();
>>>      //Protected dereference to ring->xdp_prog
>>>      xdp_prog = ring->xdp_prog;
>>>      if(unlikely(!xdp_prg)) goto unlock;
>>
>> The addition of this branch and extra deref is now slowing down the xdp
>> path compared to the current proposal.
>>
>
> Yep, but this is an unlikely condition and the critical code here is
> much smaller and it is more clear that the rcu_read_lock here meant to
> protect the ring->xdp_prog under this small xdp critical section in
> comparison to your patch where it is held across the whole RX
> function.

Note that there is already an rcu_read_lock potentially per packet
buried in the function, if the whole function is under rcu_read_lock
then that can be removed.

Tom

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