Messages in this thread | | | Subject | Re: [PATCH 1/1] mm/mmu_gather: limit tlb batch count and add schedule point in tlb_batch_pages_flush | From | wangjianxing <> | Date | Tue, 15 Mar 2022 20:56:23 +0800 |
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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@loongson.cn/
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@loongson.cn> > --- > 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; > }
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