Re: [v3 PATCH 1/2] mm: swap: check if swap backing device is congested or not

[Yang Shi <yang.shi@linux.alibaba.com>]

On 12/28/18 4:42 PM, Andrew Morton wrote:
> On Sat, 22 Dec 2018 05:40:19 +0800 Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>> Swap readahead would read in a few pages regardless if the underlying
>> device is busy or not.  It may incur long waiting time if the device is
>> congested, and it may also exacerbate the congestion.
>>
>> Use inode_read_congested() to check if the underlying device is busy or
>> not like what file page readahead does.  Get inode from swap_info_struct.
>> Although we can add inode information in swap_address_space
>> (address_space->host), it may lead some unexpected side effect, i.e.
>> it may break mapping_cap_account_dirty().  Using inode from
>> swap_info_struct seems simple and good enough.
>>
>> Just does the check in vma_cluster_readahead() since
>> swap_vma_readahead() is just used for non-rotational device which
>> much less likely has congestion than traditional HDD.
>>
>> Although swap slots may be consecutive on swap partition, it still may be
>> fragmented on swap file. This check would help to reduce excessive stall
>> for such case.
> Some words about the observed effects of the patch would be more than
> appropriate!

Yes, sure. Actually, this could reduce the latency long tail of 
do_swap_page() on a congested system.

The test on my virtual machine with emulated HDD shows:

Without swap congestion check:
page_fault1_thr-1490  [023]   129.311706: funcgraph_entry:      # 
57377.796 us |  do_swap_page();
  page_fault1_thr-1490  [023]   129.369103: funcgraph_entry: 5.642 us   
|  do_swap_page();
  page_fault1_thr-1490  [023]   129.369119: funcgraph_entry:      # 
1289.592 us |  do_swap_page();
  page_fault1_thr-1490  [023]   129.370411: funcgraph_entry: 4.957 us   
|  do_swap_page();
  page_fault1_thr-1490  [023]   129.370419: funcgraph_entry: 1.940 us   
|  do_swap_page();
  page_fault1_thr-1490  [023]   129.378847: funcgraph_entry:      # 
1411.385 us |  do_swap_page();
  page_fault1_thr-1490  [023]   129.380262: funcgraph_entry: 3.916 us   
|  do_swap_page();
  page_fault1_thr-1490  [023]   129.380275: funcgraph_entry:      # 
4287.751 us |  do_swap_page();


With swap congestion check:
       runtest.py-1417  [020]   301.925911: funcgraph_entry:      # 
9870.146 us |  do_swap_page();
       runtest.py-1417  [020]   301.935785: funcgraph_entry: 9.802 us   
|  do_swap_page();
       runtest.py-1417  [020]   301.935799: funcgraph_entry: 3.551 us   
|  do_swap_page();
       runtest.py-1417  [020]   301.935806: funcgraph_entry: 2.142 us   
|  do_swap_page();
       runtest.py-1417  [020]   301.935853: funcgraph_entry: 6.938 us   
|  do_swap_page();
       runtest.py-1417  [020]   301.935864: funcgraph_entry: 3.765 us   
|  do_swap_page();
       runtest.py-1417  [020]   301.935871: funcgraph_entry: 3.600 us   
|  do_swap_page();
       runtest.py-1417  [020]   301.935878: funcgraph_entry: 7.202 us   
|  do_swap_page();


The long tail latency (>1000us) is reduced significantly.

BTW, do you need I resend the patch with the above information appended 
into the commit log?

Thanks,
Yang