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[185.121.47.193]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a6fe5e27995sm416499666b.0.2024.06.25.13.05.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Jun 2024 13:05:53 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 25 Jun 2024 22:05:52 +0200 To: Baoquan He Cc: Nick Bowler , Hailong Liu , linux-kernel@vger.kernel.org, Linux regressions mailing list , linux-mm@kvack.org, sparclinux@vger.kernel.org, Andrew Morton Subject: Re: PROBLEM: kernel crashes when running xfsdump since ~6.4 Message-ID: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: > > > > > /** > > > > > * cpumask_next - get the next cpu in a cpumask > > > > > * @n: the cpu prior to the place to search (i.e. return will be > @n) > > > > > * @srcp: the cpumask pointer > > > > > * > > > > > * Return: >= nr_cpu_ids if no further cpus set. > > > > > > > > Ah, I got what you mean. In the vbq case, it may not have chance to get > > > > a return number as nr_cpu_ids. Becuase the hashed index limits the > > > > range to [0, nr_cpu_ids-1], and cpu_possible(index) will guarantee it > > > > won't be the highest cpu number [nr_cpu_ids-1] since CPU[nr_cpu_ids-1] must > > > > be possible CPU. > > > > > > > > Do I miss some corner cases? > > > > > > > Right. We guarantee that a highest CPU is available by doing: % nr_cpu_ids. > > > So we do not need to use *next_wrap() variant. You do not miss anything :) > > > > > > Hailong Liu has proposed more simpler version: > > > > > > > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > > > index 11fe5ea208aa..e1e63ffb9c57 100644 > > > --- a/mm/vmalloc.c > > > +++ b/mm/vmalloc.c > > > @@ -1994,8 +1994,9 @@ static struct xarray * > > > addr_to_vb_xa(unsigned long addr) > > > { > > > int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus(); > > > + int cpu = cpumask_nth(index, cpu_possible_mask); > > > > > > - return &per_cpu(vmap_block_queue, index).vmap_blocks; > > > + return &per_cpu(vmap_block_queue, cpu).vmap_blocks; > > > > > > > > > which just takes a next CPU if an index is not set in the cpu_possible_mask. > > > > > > The only thing that can be updated in the patch is to replace num_possible_cpu() > > > by the nr_cpu_ids. > > > > > > Any thoughts? I think we need to fix it by a minor change so it is > > > easier to back-port on stable kernels. > > > > Yeah, sounds good since the regresson commit is merged in v6.3. > > Please feel free to post this and the hash array patch separately for > > formal reviewing. > > > Agreed! The patch about hash array i will post later. > > > By the way, when I am replying this mail, I check the cpumask_nth() > > again. I doubt it may take more checking then cpu_possible(), given most > > of systems don't have gaps in cpu_possible_mask. I could be dizzy at > > this moment. > > > > static inline unsigned int cpumask_nth(unsigned int cpu, const struct cpumask *srcp) > > { > > return find_nth_bit(cpumask_bits(srcp), small_cpumask_bits, cpumask_check(cpu)); > > } > > > Yep, i do not think it is a big problem based on your noted fact. > Checked. There is a difference: 1. Default ... + 15.95% 6.05% [kernel] [k] __vmap_pages_range_noflush + 15.91% 1.74% [kernel] [k] addr_to_vb_xa <--------------- + 15.13% 12.05% [kernel] [k] vunmap_p4d_range + 14.17% 13.38% [kernel] [k] __find_nth_bit <-------------- + 10.62% 0.00% [kernel] [k] ret_from_fork_asm + 10.62% 0.00% [kernel] [k] ret_from_fork + 10.62% 0.00% [kernel] [k] kthread ... 2. Check if cpu_possible() and then fallback to cpumask_nth() if not ... + 6.84% 0.29% [kernel] [k] alloc_vmap_area + 6.80% 6.70% [kernel] [k] native_queued_spin_lock_slowpath + 4.24% 0.09% [kernel] [k] free_vmap_block + 2.41% 2.38% [kernel] [k] addr_to_vb_xa <----------- + 1.94% 1.91% [kernel] [k] xas_start ... It is _worth_ to check if an index is in possible mask: diff --git a/mm/vmalloc.c b/mm/vmalloc.c index 45e1506d58c3..af20f78c2cbf 100644 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c @@ -2542,7 +2542,10 @@ static DEFINE_PER_CPU(struct vmap_block_queue, vmap_block_queue); static struct xarray * addr_to_vb_xa(unsigned long addr) { - int index = (addr / VMAP_BLOCK_SIZE) % num_possible_cpus(); + int index = (addr / VMAP_BLOCK_SIZE) % nr_cpu_ids; + + if (!cpu_possible(index)) + index = cpumask_nth(index, cpu_possible_mask); return &per_cpu(vmap_block_queue, index).vmap_blocks; } cpumask_nth() is not cheap. My measurements are based on a synthetic tight test and it detects a difference. In a real workloads it should not be visible. Having gaps is not a common case plus a "slow path" will be mitigated by the hit against possible mask. -- Uladzislau Rezki