linux-kernel - Re: [PATCH v3 00/11] Mitigate a vmap lock contention v3

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Message-ID: <Zdh8KmZtoQym7Syz@pc636>
Date: Fri, 23 Feb 2024 12:06:18 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>
Cc: Uladzislau Rezki <urezki@...il.com>,
	Pedro Falcato <pedro.falcato@...il.com>,
	Matthew Wilcox <willy@...radead.org>, Mel Gorman <mgorman@...e.de>,
	kirill.shutemov@...ux.intel.com,
	Vishal Moola <vishal.moola@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
	linux-mm@...ck.org
Subject: Re: [PATCH v3 00/11] Mitigate a vmap lock contention v3

> On 02/23/24 at 10:34am, Uladzislau Rezki wrote:
> > On Thu, Feb 22, 2024 at 11:15:59PM +0000, Pedro Falcato wrote:
> > > Hi,
> > > 
> > > On Thu, Feb 22, 2024 at 8:35 AM Uladzislau Rezki <urezki@...il.com> wrote:
> > > >
> > > > Hello, Folk!
> > > >
> > > >[...]
> > > > pagetable_alloc - gets increased as soon as a higher pressure is applied by
> > > > increasing number of workers. Running same number of jobs on a next run
> > > > does not increase it and stays on same level as on previous.
> > > >
> > > > /**
> > > >  * pagetable_alloc - Allocate pagetables
> > > >  * @gfp:    GFP flags
> > > >  * @order:  desired pagetable order
> > > >  *
> > > >  * pagetable_alloc allocates memory for page tables as well as a page table
> > > >  * descriptor to describe that memory.
> > > >  *
> > > >  * Return: The ptdesc describing the allocated page tables.
> > > >  */
> > > > static inline struct ptdesc *pagetable_alloc(gfp_t gfp, unsigned int order)
> > > > {
> > > >         struct page *page = alloc_pages(gfp | __GFP_COMP, order);
> > > >
> > > >         return page_ptdesc(page);
> > > > }
> > > >
> > > > Could you please comment on it? Or do you have any thought? Is it expected?
> > > > Is a page-table ever shrink?
> > > 
> > > It's my understanding that the vunmap_range helpers don't actively
> > > free page tables, they just clear PTEs. munmap does free them in
> > > mmap.c:free_pgtables, maybe something could be worked up for vmalloc
> > > too.
> > >
> > Right. I see that for a user space, pgtables are removed. There was a
> > work on it.
> > 
> > >
> > > I would not be surprised if the memory increase you're seeing is more
> > > or less correlated to the maximum vmalloc footprint throughout the
> > > whole test.
> > > 
> > Yes, the vmalloc footprint follows the memory usage. Some uses cases
> > map lot of memory.
> 
> The 'nr_threads=256' testing may be too radical. I took the test on
> a bare metal machine as below, it's still running and hang there after
> 30 minutes. I did this after system boot. I am looking for other
> machines with more processors.
> 
> [root@...l-r640-068 ~]# nproc 
> 64
> [root@...l-r640-068 ~]# free -h
>                total        used        free      shared  buff/cache   available
> Mem:           187Gi        18Gi       169Gi        12Mi       262Mi       168Gi
> Swap:          4.0Gi          0B       4.0Gi
> [root@...l-r640-068 ~]# 
> 
> [root@...l-r640-068 linux]# tools/testing/selftests/mm/test_vmalloc.sh run_test_mask=127 nr_threads=256
> Run the test with following parameters: run_test_mask=127 nr_threads=256
> 
Agree, nr_threads=256 is a way radical :) Mine took 50 minutes to
complete. So wait more :)


--
Uladzislau Rezki