linux-kernel - [PATCH v2 1/1] mm/mmu_gather: limit free batch count and add schedule point in tlb_batch_pages

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Date:   Thu, 17 Mar 2022 03:28:57 -0400
From:   Jianxing Wang <wangjianxing@...ngson.cn>
To:     peterz@...radead.org
Cc:     will@...nel.org, aneesh.kumar@...ux.ibm.com,
        akpm@...ux-foundation.org, npiggin@...il.com,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Jianxing Wang <wangjianxing@...ngson.cn>
Subject: [PATCH v2 1/1] mm/mmu_gather: limit free batch count and add schedule point in tlb_batch_pages_flush

free a large list of pages maybe cause rcu_sched starved on
non-preemptible kernels. howerver free_unref_page_list maybe can't
cond_resched as it maybe called in interrupt or atomic context,
especially can't detect atomic context in CONFIG_PREEMPTION=n.

tlb flush batch count depends on PAGE_SIZE, it's too large if
PAGE_SIZE > 4K, here limit free batch count with 512.
And add schedule point in tlb_batch_pages_flush.

rcu: rcu_sched kthread starved for 5359 jiffies! g454793 f0x0
RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=19
[...]
Call Trace:
   free_unref_page_list+0x19c/0x270
   release_pages+0x3cc/0x498
   tlb_flush_mmu_free+0x44/0x70
   zap_pte_range+0x450/0x738
   unmap_page_range+0x108/0x240
   unmap_vmas+0x74/0xf0
   unmap_region+0xb0/0x120
   do_munmap+0x264/0x438
   vm_munmap+0x58/0xa0
   sys_munmap+0x10/0x20
   syscall_common+0x24/0x38

Signed-off-by: Jianxing Wang <wangjianxing@...ngson.cn>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>

---
ChangeLog:
V1 -> V2: limit free batch count directly in tlb_batch_pages_flush
---
---
 mm/mmu_gather.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index afb7185ffdc4..a71924bd38c0 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -47,8 +47,20 @@ static void tlb_batch_pages_flush(struct mmu_gather *tlb)
 	struct mmu_gather_batch *batch;

 	for (batch = &tlb->local; batch && batch->nr; batch = batch->next) {
-		free_pages_and_swap_cache(batch->pages, batch->nr);
-		batch->nr = 0;
+		struct page **pages = batch->pages;
+
+		do {
+			/*
+			 * limit free batch count when PAGE_SIZE > 4K
+			 */
+			unsigned int nr = min(512U, batch->nr);
+
+			free_pages_and_swap_cache(pages, nr);
+			pages += nr;
+			batch->nr -= nr;
+
+			cond_resched();
+		} while (batch->nr);
 	}
 	tlb->active = &tlb->local;
 }
-- 
2.31.1