Re: [v5 PATCH 1/2] mm: swap: check if swap backing device is congested or not

[Yang Shi]

Hi Andrew,


How do you look these patches? They had been reviewed and the commit log 
has been updated per your and Daniel's comments.


Thanks,

Yang



On 1/3/19 11:27 AM, Yang Shi 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.
>
> The test with page_fault1 of will-it-scale (sometimes tracing may just
> show runtest.py that is the wrapper script of page_fault1), which basically
> launches NR_CPU threads to generate 128MB anonymous pages for each thread,
> on my virtual machine with congested HDD shows long tail latency is reduced
> significantly.
>
> Without the patch
>   page_fault1_thr-1490  [023]   129.311706: funcgraph_entry:      #57377.796 us |  do_swap_page();
>   page_fault1_thr-1490  [023]   129.369103: funcgraph_entry:        5.642us   |  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:        4.957us   |  do_swap_page();
>   page_fault1_thr-1490  [023]   129.370419: funcgraph_entry:        1.940us   |  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:        3.916us   |  do_swap_page();
>   page_fault1_thr-1490  [023]   129.380275: funcgraph_entry:      #4287.751 us |  do_swap_page();
>
> With the patch
>        runtest.py-1417  [020]   301.925911: funcgraph_entry:      #9870.146 us |  do_swap_page();
>        runtest.py-1417  [020]   301.935785: funcgraph_entry:        9.802us   |  do_swap_page();
>        runtest.py-1417  [020]   301.935799: funcgraph_entry:        3.551us   |  do_swap_page();
>        runtest.py-1417  [020]   301.935806: funcgraph_entry:        2.142us   |  do_swap_page();
>        runtest.py-1417  [020]   301.935853: funcgraph_entry:        6.938us   |  do_swap_page();
>        runtest.py-1417  [020]   301.935864: funcgraph_entry:        3.765us   |  do_swap_page();
>        runtest.py-1417  [020]   301.935871: funcgraph_entry:        3.600us   |  do_swap_page();
>        runtest.py-1417  [020]   301.935878: funcgraph_entry:        7.202us   |  do_swap_page();
>
> Acked-by: Tim Chen <tim.c.chen@xxxxxxxxx>
> Cc: Huang Ying <ying.huang@xxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
> v5: Elaborate more about the test case per Daniel
> v4: Added observed effects in the commit log per Andrew
> v3: Move inode deference under swap device type check per Tim Chen
> v2: Check the swap device type per Tim Chen
>
>   mm/swap_state.c | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index fd2f21e..78d500e 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -538,11 +538,18 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>   	bool do_poll = true, page_allocated;
>   	struct vm_area_struct *vma = vmf->vma;
>   	unsigned long addr = vmf->address;
> +	struct inode *inode = NULL;
>   
>   	mask = swapin_nr_pages(offset) - 1;
>   	if (!mask)
>   		goto skip;
>   
> +	if (si->flags & (SWP_BLKDEV | SWP_FS)) {
> +		inode = si->swap_file->f_mapping->host;
> +		if (inode_read_congested(inode))
> +			goto skip;
> +	}
> +
>   	do_poll = false;
>   	/* Read a page_cluster sized and aligned cluster around offset. */
>   	start_offset = offset & ~mask;