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

[Olof Johansson]

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
     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.


-Olof