netdev - Re: Regression in throughput between kvm guests over virtual bridge

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Message-ID: <55f9173b-a419-98f0-2516-cbd57299ba5d@redhat.com>
Date:   Mon, 18 Sep 2017 11:13:58 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.vnet.ibm.com>,
        netdev@...r.kernel.org
Cc:     davem@...emloft.net, mst@...hat.com
Subject: Re: Regression in throughput between kvm guests over virtual bridge



On 2017年09月16日 03:19, Matthew Rosato wrote:
>> It looks like vhost is slowed down for some reason which leads to more
>> idle time on 4.13+VHOST_RX_BATCH=1. Appreciated if you can collect the
>> perf.diff on host, one for rx and one for tx.
>>
> perf data below for the associated vhost threads, baseline=4.12,
> delta1=4.13, delta2=4.13+VHOST_RX_BATCH=1
>
> Client vhost:
>
> 60.12%  -11.11%  -12.34%  [kernel.vmlinux]   [k] raw_copy_from_user
> 13.76%   -1.28%   -0.74%  [kernel.vmlinux]   [k] get_page_from_freelist
>   2.00%   +3.69%   +3.54%  [kernel.vmlinux]   [k] __wake_up_sync_key
>   1.19%   +0.60%   +0.66%  [kernel.vmlinux]   [k] __alloc_pages_nodemask
>   1.12%   +0.76%   +0.86%  [kernel.vmlinux]   [k] copy_page_from_iter
>   1.09%   +0.28%   +0.35%  [vhost]            [k] vhost_get_vq_desc
>   1.07%   +0.31%   +0.26%  [kernel.vmlinux]   [k] alloc_skb_with_frags
>   0.94%   +0.42%   +0.65%  [kernel.vmlinux]   [k] alloc_pages_current
>   0.91%   -0.19%   -0.18%  [kernel.vmlinux]   [k] memcpy
>   0.88%   +0.26%   +0.30%  [kernel.vmlinux]   [k] __next_zones_zonelist
>   0.85%   +0.05%   +0.12%  [kernel.vmlinux]   [k] iov_iter_advance
>   0.79%   +0.09%   +0.19%  [vhost]            [k] __vhost_add_used_n
>   0.74%                    [kernel.vmlinux]   [k] get_task_policy.part.7
>   0.74%   -0.01%   -0.05%  [kernel.vmlinux]   [k] tun_net_xmit
>   0.60%   +0.17%   +0.33%  [kernel.vmlinux]   [k] policy_nodemask
>   0.58%   -0.15%   -0.12%  [ebtables]         [k] ebt_do_table
>   0.52%   -0.25%   -0.22%  [kernel.vmlinux]   [k] __alloc_skb
>     ...
>   0.42%   +0.58%   +0.59%  [kernel.vmlinux]   [k] eventfd_signal
>     ...
>   0.32%   +0.96%   +0.93%  [kernel.vmlinux]   [k] finish_task_switch
>     ...
>           +1.50%   +1.16%  [kernel.vmlinux]   [k] get_task_policy.part.9
>           +0.40%   +0.42%  [kernel.vmlinux]   [k] __skb_get_hash_symmetr
>           +0.39%   +0.40%  [kernel.vmlinux]   [k] _copy_from_iter_full
>           +0.24%   +0.23%  [vhost_net]        [k] vhost_net_buf_peek
>
> Server vhost:
>
> 61.93%  -10.72%  -10.91%  [kernel.vmlinux]   [k] raw_copy_to_user
>   9.25%   +0.47%   +0.86%  [kernel.vmlinux]   [k] free_hot_cold_page
>   5.16%   +1.41%   +1.57%  [vhost]            [k] vhost_get_vq_desc
>   5.12%   -3.81%   -3.78%  [kernel.vmlinux]   [k] skb_release_data
>   3.30%   +0.42%   +0.55%  [kernel.vmlinux]   [k] raw_copy_from_user
>   1.29%   +2.20%   +2.28%  [kernel.vmlinux]   [k] copy_page_to_iter
>   1.24%   +1.65%   +0.45%  [vhost_net]        [k] handle_rx
>   1.08%   +3.03%   +2.85%  [kernel.vmlinux]   [k] __wake_up_sync_key
>   0.96%   +0.70%   +1.10%  [vhost]            [k] translate_desc
>   0.69%   -0.20%   -0.22%  [kernel.vmlinux]   [k] tun_do_read.part.10
>   0.69%                    [kernel.vmlinux]   [k] tun_peek_len
>   0.67%   +0.75%   +0.78%  [kernel.vmlinux]   [k] eventfd_signal
>   0.52%   +0.96%   +0.98%  [kernel.vmlinux]   [k] finish_task_switch
>   0.50%   +0.05%   +0.09%  [vhost]            [k] vhost_add_used_n
>     ...
>           +0.63%   +0.58%  [vhost_net]        [k] vhost_net_buf_peek
>           +0.32%   +0.32%  [kernel.vmlinux]   [k] _copy_to_iter
>           +0.19%   +0.19%  [kernel.vmlinux]   [k] __skb_get_hash_symmetr
>           +0.11%   +0.21%  [vhost]            [k] vhost_umem_interval_tr
>

Looks like for some unknown reason which leads more wakeups.

Could you please try to attached patch to see if it solves or mitigate 
the issue?

Thanks

View attachment "0001-vhost_net-conditionally-enable-tx-polling.patch" of type "text/x-patch" (900 bytes)