Date:   Fri, 2 Sep 2016 01:59:40 +0300
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Brenden Blanco <bblanco@...mgrid.com>
Cc:     Tom Herbert <tom@...bertland.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 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.

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