Re: [PATCH] net/sock: move memory_allocated over to percpu_counter variables

[Eric Dumazet]

On Sat, Sep 8, 2018 at 10:02 AM Olof Johansson <olof@xxxxxxxxx> wrote:
>
> Hi,
>
> On Fri, Sep 7, 2018 at 12:21 AM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > On Fri, Sep 7, 2018 at 12:03 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> >> Problem is : we have platforms with more than 100 cpus, and
> >> sk_memory_allocated() cost will be too expensive,
> >> especially if the host is under memory pressure, since all cpus will
> >> touch their private counter.
> >>
> >> per cpu variables do not really scale, they were ok 10 years ago when
> >> no more than 16 cpus were the norm.
> >>
> >> I would prefer change TCP to not aggressively call
> >> __sk_mem_reduce_allocated() from tcp_write_timer()
> >>
> >> Ideally only tcp_retransmit_timer() should attempt to reduce forward
> >> allocations, after recurring timeout.
> >>
> >> Note that after 20c64d5cd5a2bdcdc8982a06cb05e5e1bd851a3d ("net: avoid
> >> sk_forward_alloc overflows")
> >> we have better control over sockets having huge forward allocations.
> >>
> >> Something like :
> >
> > Or something less risky :
>
> I gave both of these patches a run, and neither do as well on the
> system that has slower atomics. :(
>
> The percpu version:
>
>      8.05%  workload         [kernel.vmlinux]
>     [k] __do_softirq
>      7.04%  swapper          [kernel.vmlinux]
>     [k] cpuidle_enter_state
>      5.54%  workload         [kernel.vmlinux]
>     [k] _raw_spin_unlock_irqrestore
>      1.66%  swapper          [kernel.vmlinux]
>     [k] __do_softirq
>      1.55%  workload         [kernel.vmlinux]
>     [k] finish_task_switch
>      1.24%  swapper          [kernel.vmlinux]
>     [k] finish_task_switch
>      1.07%  workload         [kernel.vmlinux]
>     [k] net_rx_action
>
> The first patch from you still has significant amount of time spent in
> the atomics paths (non-inlined versions used):
>
>      7.87%  workload         [kernel.vmlinux]
> [k] __ll_sc_atomic64_sub


The second patch I gave should not enter this path at all, please try it.

>      7.48%  workload         [kernel.vmlinux]
> [k] __do_softirq
>      5.05%  workload         [kernel.vmlinux]
> [k] _raw_spin_unlock_irqrestore
>      2.42%  workload         [kernel.vmlinux]
> [k] __ll_sc_atomic64_add_return
>      1.49%  swapper          [kernel.vmlinux]
> [k] cpuidle_enter_state
>      1.31%  workload         [kernel.vmlinux]
> [k] finish_task_switch
>      1.09%  workload         [kernel.vmlinux]
> [k] tcp_sendmsg_locked
>      1.08%  workload         [kernel.vmlinux]
> [k] __arch_copy_from_user
>      1.02%  workload         [kernel.vmlinux]
> [k] net_rx_action
>
> I think a lot of the overhead from percpu approach can be alleviated
> if we can use percpu_counter_read() instead of _sum() (i.e. no need to
> iterate through the local per-cpu recent delta). I don't know the TCP
> stack well enough to tell where it's OK to use a bit of slack in the
> numbers though -- by default count will at most be off by 32*online
> cpus. Might not be a significant number in reality.