linux-kernel - Re: [PATCH v6] mm/vmalloc: lock contention optimization under multi-threading

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Date: Thu, 29 Feb 2024 11:33:24 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Baoquan He <bhe@...hat.com>, rulinhuang <rulin.huang@...el.com>
Cc: rulinhuang <rulin.huang@...el.com>, urezki@...il.com,
	akpm@...ux-foundation.org, colin.king@...el.com, hch@...radead.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	lstoakes@...il.com, tianyou.li@...el.com, tim.c.chen@...el.com,
	wangyang.guo@...el.com, zhiguo.zhou@...el.com
Subject: Re: [PATCH v6] mm/vmalloc: lock contention optimization under
 multi-threading

On Thu, Feb 29, 2024 at 06:12:00PM +0800, Baoquan He wrote:
> Hi Rulin,
> 
> Thanks for the great work and v6, some concerns, please see inline
> comments.
> 
> On 02/29/24 at 12:26am, rulinhuang wrote:
> > When allocating a new memory area where the mapping address range is
> > known, it is observed that the vmap_node->busy.lock is acquired twice.
> > 
> > The first acquisition occurs in the alloc_vmap_area() function when
> > inserting the vm area into the vm mapping red-black tree. The second
> > acquisition occurs in the setup_vmalloc_vm() function when updating the
> > properties of the vm, such as flags and address, etc.
> > 
> > Combine these two operations together in alloc_vmap_area(), which
> > improves scalability when the vmap_node->busy.lock is contended.
> > By doing so, the need to acquire the lock twice can also be eliminated
> >  to once.
> > 
> > With the above change, tested on intel sapphire rapids
> > platform(224 vcpu), a 4% performance improvement is
> > gained on stress-ng/pthread(https://github.com/ColinIanKing/stress-ng),
> > which is the stress test of thread creations.
> > 
> > Reviewed-by: Uladzislau Rezki <urezki@...il.com>
> > Reviewed-by: Baoquan He <bhe@...hat.com>
> > Reviewed-by: "Chen, Tim C" <tim.c.chen@...el.com>
> > Reviewed-by: "King, Colin" <colin.king@...el.com>
> 
> 
> We possibly need remove these reviewers' tags when new code change is
> taken so that people check and add Acked-by or Reviewed-by again if then
> agree, or add new comments if any concern.
> 
> > Signed-off-by: rulinhuang <rulin.huang@...el.com>
> > ---
> > V1 -> V2: Avoided the partial initialization issue of vm and
> > separated insert_vmap_area() from alloc_vmap_area()
> > V2 -> V3: Rebased on 6.8-rc5
> > V3 -> V4: Rebased on mm-unstable branch
> > V4 -> V5: cancel the split of alloc_vmap_area()
> > and keep insert_vmap_area()
> > V5 -> V6: add bug_on
> > ---
> >  mm/vmalloc.c | 132 +++++++++++++++++++++++++--------------------------
> >  1 file changed, 64 insertions(+), 68 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 25a8df497255..5ae028b0d58d 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1841,15 +1841,66 @@ node_alloc(unsigned long size, unsigned long align,
> >  	return va;
> >  }
> >  
> > +/*** Per cpu kva allocator ***/
> > +
> > +/*
> > + * vmap space is limited especially on 32 bit architectures. Ensure there is
> > + * room for at least 16 percpu vmap blocks per CPU.
> > + */
> > +/*
> > + * If we had a constant VMALLOC_START and VMALLOC_END, we'd like to be able
> > + * to #define VMALLOC_SPACE		(VMALLOC_END-VMALLOC_START). Guess
> > + * instead (we just need a rough idea)
> > + */
> > +#if BITS_PER_LONG == 32
> > +#define VMALLOC_SPACE		(128UL*1024*1024)
> > +#else
> > +#define VMALLOC_SPACE		(128UL*1024*1024*1024)
> > +#endif
> > +
> > +#define VMALLOC_PAGES		(VMALLOC_SPACE / PAGE_SIZE)
> > +#define VMAP_MAX_ALLOC		BITS_PER_LONG	/* 256K with 4K pages */
> > +#define VMAP_BBMAP_BITS_MAX	1024	/* 4MB with 4K pages */
> > +#define VMAP_BBMAP_BITS_MIN	(VMAP_MAX_ALLOC*2)
> > +#define VMAP_MIN(x, y)		((x) < (y) ? (x) : (y)) /* can't use min() */
> > +#define VMAP_MAX(x, y)		((x) > (y) ? (x) : (y)) /* can't use max() */
> > +#define VMAP_BBMAP_BITS		\
> > +		VMAP_MIN(VMAP_BBMAP_BITS_MAX,	\
> > +		VMAP_MAX(VMAP_BBMAP_BITS_MIN,	\
> > +			VMALLOC_PAGES / roundup_pow_of_two(NR_CPUS) / 16))
> > +
> > +#define VMAP_BLOCK_SIZE		(VMAP_BBMAP_BITS * PAGE_SIZE)
> > +
> > +/*
> > + * Purge threshold to prevent overeager purging of fragmented blocks for
> > + * regular operations: Purge if vb->free is less than 1/4 of the capacity.
> > + */
> > +#define VMAP_PURGE_THRESHOLD	(VMAP_BBMAP_BITS / 4)
> > +
> > +#define VMAP_RAM		0x1 /* indicates vm_map_ram area*/
> > +#define VMAP_BLOCK		0x2 /* mark out the vmap_block sub-type*/
> > +#define VMAP_FLAGS_MASK		0x3
> 
> These code moving is made because we need check VMAP_RAM in advance. We
> may need move all those data structures and basic helpers related to per
> cpu kva allocator up too to along with these macros, just as the newly
> introduced vmap_node does. If that's agreed, better be done in a
> separate patch. My personal opinion. Not sure if Uladzislau has
> different thoughts.
> 
> Other than this, the overall looks good to me.
> 
I agree, the split should be done. One is a preparation move saying that
no functional change happens and final one an actual change is.

--
Uladzislau Rezki