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

Yang Shi
2019-01-10T20:52:08Z

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@intel.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> ---
> 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;