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Wed, 26 Jun 2024 00:39:00 +0000 (UTC) Date: Wed, 26 Jun 2024 08:38:56 +0800 From: Baoquan He To: Uladzislau Rezki 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: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 X-Rspamd-Queue-Id: 9B7AB100012 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 1bjc8pu6pfuqestohd8scqu1yjfjf4qe X-HE-Tag: 1719362350-505118 X-HE-Meta: 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 PiK1LL/8 jeotDK7zWsRXXwgQWfDEYCODr1cbiWpu7pq3qYQYCoQgwSe3TJEn7P5VzQ8XJ/FwW1LWjnhoFwWBkTPgz5I/lzyqhDSbb5L2oK0dXJsyyQIXWqZpxsVSwaTzC90xHWf6nwz0Mx5vjY3q96kNnDuDjbFwzF0EBFBIm/7anbwF8KQPcPo+RaskxhPnR81tTVbvJkWd/wSTqBoDkPCNGqfNjGiMgbOPa8HRFYanwup4gXFEyyCFrBDavi+00zJnT5TusDNUPdXIwO/Rslm0TNkYwNtyh0JRkUghyNW5IINeqp3O+N6MFByx1MOEiLvjMtbHdwbNVW30JDw/8SeU= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 06/25/24 at 10:05pm, Uladzislau Rezki wrote: > > > > > > /** > > > > > > * 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. Ah, this is consistent with my understanding from the code, thanks for confirming by testing.