1. I try to increase the overcommit ratio of cpu to 1:2~1:3 in KVM
hypervisor, per-vm has the same number of vcpu with host cpu, then setup
2 or 3 vm.
2. Run ltpstress(20180926) test in per vm, both host and guest is
non-preemptiable
kernel, vm dmesg will throw some rcu_sched warning.
3. PAGE_SIZE in my kernel config is 16K, tlb batch max count is 2015,
it's too long in non-preemptible state.
The issue's orignal
link:https://patchwork.kernel.org/project/linux-mm/patch/20220302013825.2290315-1-wangjianxing@xxxxxxxxxxx/
On 03/15/2022 08:55 PM, Jianxing Wang wrote:
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 max batch size with 4K.
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@xxxxxxxxxxx>
---
include/asm-generic/tlb.h | 7 ++++++-
mm/mmu_gather.c | 7 +++++--
2 files changed, 11 insertions(+), 3 deletions(-)
diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 2c68a545ffa7..47c7f93ca695 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -230,8 +230,13 @@ struct mmu_gather_batch {
struct page *pages[0];
};
+#if PAGE_SIZE > 4096UL
+#define MAX_GATHER_BATCH_SZ 4096
+#else
+#define MAX_GATHER_BATCH_SZ PAGE_SIZE
+#endif
#define MAX_GATHER_BATCH \
- ((PAGE_SIZE - sizeof(struct mmu_gather_batch)) / sizeof(void *))
+ ((MAX_GATHER_BATCH_SZ - sizeof(struct mmu_gather_batch)) / sizeof(void *))
/*
* Limit the maximum number of mmu_gather batches to reduce a risk of soft
diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index afb7185ffdc4..f2c105810b3f 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -8,6 +8,7 @@
#include <linux/rcupdate.h>
#include <linux/smp.h>
#include <linux/swap.h>
+#include <linux/slab.h>
#include <asm/pgalloc.h>
#include <asm/tlb.h>
@@ -27,7 +28,7 @@ static bool tlb_next_batch(struct mmu_gather *tlb)
if (tlb->batch_count == MAX_GATHER_BATCH_COUNT)
return false;
- batch = (void *)__get_free_pages(GFP_NOWAIT | __GFP_NOWARN, 0);
+ batch = kmalloc(MAX_GATHER_BATCH_SZ, GFP_NOWAIT | __GFP_NOWARN);
if (!batch)
return false;
@@ -49,6 +50,8 @@ static void tlb_batch_pages_flush(struct mmu_gather *tlb)
for (batch = &tlb->local; batch && batch->nr; batch = batch->next) {
free_pages_and_swap_cache(batch->pages, batch->nr);
batch->nr = 0;
+
+ cond_resched();
}
tlb->active = &tlb->local;
}
@@ -59,7 +62,7 @@ static void tlb_batch_list_free(struct mmu_gather *tlb)
for (batch = tlb->local.next; batch; batch = next) {
next = batch->next;
- free_pages((unsigned long)batch, 0);
+ kfree(batch);
}
tlb->local.next = NULL;
}
