Re: [RFC PATCH v1 12/13] mm: split up release_pages into non-sentinel and sentinel passes

[Laurent Dufour]

On 01/02/2018 00:04, daniel.m.jordan@xxxxxxxxxx wrote:
> A common case in release_pages is for the 'pages' list to be in roughly
> the same order as they are in their LRU.  With LRU batch locking, when a
> sentinel page is removed, an adjacent non-sentinel page must be promoted
> to a sentinel page to follow the locking scheme.  So we can get behavior
> where nearly every page in the 'pages' array is treated as a sentinel
> page, hurting the scalability of this approach.
> 
> To address this, split up release_pages into non-sentinel and sentinel
> passes so that the non-sentinel pages can be locked with an LRU batch
> lock before the sentinel pages are removed.
> 
> For the prototype, just use a bitmap and a temporary outer loop to
> implement this.
> 
> Performance numbers from a single microbenchmark at this point in the
> series are included in the next patch.
> 
> Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> ---
>  mm/swap.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index fae766e035a4..a302224293ad 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -731,6 +731,7 @@ void lru_add_drain_all(void)
>  	put_online_cpus();
>  }
> 
> +#define LRU_BITMAP_SIZE	512
>  /**
>   * release_pages - batched put_page()
>   * @pages: array of pages to release
> @@ -742,16 +743,32 @@ void lru_add_drain_all(void)
>   */
>  void release_pages(struct page **pages, int nr)
>  {
> -	int i;
> +	int h, i;
>  	LIST_HEAD(pages_to_free);
>  	struct pglist_data *locked_pgdat = NULL;
>  	spinlock_t *locked_lru_batch = NULL;
>  	struct lruvec *lruvec;
>  	unsigned long uninitialized_var(flags);
> +	DECLARE_BITMAP(lru_bitmap, LRU_BITMAP_SIZE);
> +
> +	VM_BUG_ON(nr > LRU_BITMAP_SIZE);

While running your series rebased on v4.15-mmotm-2018-01-31-16-51, I'm
hitting this VM_BUG sometimes on a ppc64 system where page size is set to 64K.
In my case, nr=537 while LRU_BITMAP_SIZE is 512. Here is the stack trace
displayed :

kernel BUG at /local/laurent/work/glinux/mm/swap.c:728!
Oops: Exception in kernel mode, sig: 5 [#1]
LE SMP NR_CPUS=2048 NUMA pSeries
Modules linked in: pseries_rng rng_core vmx_crypto virtio_balloon ip_tables
x_tables autofs4 virtio_net virtio_blk virtio_pci virtio_ring virtio
CPU: 41 PID: 3485 Comm: cc1 Not tainted 4.15.0-mm1-lru+ #2
NIP:  c0000000002b0784 LR: c0000000002b0780 CTR: c0000000007bab20
REGS: c0000005e126b740 TRAP: 0700   Not tainted  (4.15.0-mm1-lru+)
MSR:  8000000000029033 <SF,EE,ME,IR,DR,RI,LE>  CR: 28002422  XER: 20000000
CFAR: c000000000192ae4 SOFTE: 0
GPR00: c0000000002b0780 c0000005e126b9c0 c00000000103c100 000000000000001c
GPR04: c0000005ffc4ce38 c0000005ffc63d00 0000000000000000 0000000000000001
GPR08: 0000000000000007 c000000000ec3a4c 00000005fed90000 0000000000000000
GPR12: 0000000000002200 c00000000fd8cd00 0000000000000000 0000000000000000
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR24: c0000005e11ab980 0000000000000000 0000000000000000 c0000005e126ba60
GPR28: 0000000000000219 c0000005e126bc40 0000000000000000 c0000005ec5f0000
NIP [c0000000002b0784] release_pages+0x864/0x880
LR [c0000000002b0780] release_pages+0x860/0x880
Call Trace:
[c0000005e126b9c0] [c0000000002b0780] release_pages+0x860/0x880 (unreliable)
[c0000005e126bb30] [c00000000031da3c] free_pages_and_swap_cache+0x11c/0x150
[c0000005e126bb80] [c0000000002ef5f8] tlb_flush_mmu_free+0x68/0xa0
[c0000005e126bbc0] [c0000000002f1568] arch_tlb_finish_mmu+0x58/0xf0
[c0000005e126bbf0] [c0000000002f19d4] tlb_finish_mmu+0x34/0x60
[c0000005e126bc20] [c0000000003031e8] exit_mmap+0xd8/0x1d0
[c0000005e126bce0] [c0000000000f3188] mmput+0x78/0x160
[c0000005e126bd10] [c0000000000ff568] do_exit+0x348/0xd00
[c0000005e126bdd0] [c0000000000fffd8] do_group_exit+0x58/0xd0
[c0000005e126be10] [c00000000010006c] SyS_exit_group+0x1c/0x20
[c0000005e126be30] [c00000000000ba60] system_call+0x58/0x6c
Instruction dump:
3949ffff 4bfffdc8 3c62ffce 38a00200 f9c100e0 f9e100e8 386345e8 fa0100f0
fa2100f8 fa410100 4bee2329 60000000 <0fe00000> 3b400001 4bfff868 7d5d5378
---[ end trace 55b1651f9d92f14f ]---

> 
> +	bitmap_zero(lru_bitmap, nr);
> +
> +	for (h = 0; h < 2; h++) {
>  	for (i = 0; i < nr; i++) {
>  		struct page *page = pages[i];
> 
> +		if (h == 0) {
> +			if (PageLRU(page) && page->lru_sentinel) {
> +				bitmap_set(lru_bitmap, i, 1);
> +				continue;
> +			}
> +		} else {
> +			if (!test_bit(i, lru_bitmap))
> +				continue;
> +		}
> +
>  		if (is_huge_zero_page(page))
>  			continue;
> 
> @@ -798,6 +815,7 @@ void release_pages(struct page **pages, int nr)
> 
>  		list_add(&page->lru, &pages_to_free);
>  	}
> +	}
>  	if (locked_lru_batch) {
>  		lru_batch_unlock(NULL, &locked_lru_batch, &locked_pgdat,
>  				 &flags);
>