From: Uladzislau Rezki <urezki@gmail.com>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: ksummit-discuss@lists.linuxfoundation.org
Subject: Re: [Ksummit-discuss] [TECH TOPIC] Reworking of KVA allocator in Linux kernel
Date: Sun, 2 Jun 2019 20:31:30 +0200 [thread overview]
Message-ID: <20190602183130.6wjpst7iq6g4fehr@pc636> (raw)
In-Reply-To: <20190602120510.sivqftjj6fg7s5q3@pc636>
On Sun, Jun 02, 2019 at 02:05:10PM +0200, Uladzislau Rezki wrote:
> Hello, Matthew.
>
> >
> > Vlad, I was under the impression this work was complete.
> >
> Thank you. Actually there was a discussion once upon a time:
>
> <snip>
> I think our real problem is that we have no data structure that stores
> free VA space. We have the vmap_area which stores allocated space, but no
> data structure to store free space.
> <snip>
>
> and it was a good argument to start to examine the KVA and its problems :)
>
> >
> > Are there any remaining issues to discuss?
> >
> If we have a look at it from issues point of view, then i do not see them.
> Though, there are some small things i would like to refactor. For instance
> see below:
>
> https://lkml.org/lkml/2019/5/28/1040
>
> Apart from that, there is still a window for improvements. As an example
> i would like to reduce a lock contention. In general it means making of
> the entire logic faster or/+ reworking of locking. Below the perf output
> in case of stressing my box(Intel Xeon 6 physical CPUs, ~3,8Ghz) by running
> 6 simultaneous pinned jobs which do random allocations:
>
> <snip>
> 49.55% [kernel] [k] native_queued_spin_lock_slowpath
> 7.49% [kernel] [k] get_page_from_freelist
> 4.65% [kernel] [k] alloc_vmap_area
> 4.15% [kernel] [k] _raw_spin_lock
> 4.15% [kernel] [k] free_unref_page
> 2.80% [kernel] [k] __alloc_pages_nodemask
> 2.53% [kernel] [k] insert_vmap_area.constprop.48
> 2.47% [kernel] [k] vunmap_page_range
> 2.10% [kernel] [k] __free_pages
> 1.79% [kernel] [k] find_vmap_area
> 1.66% [kernel] [k] vmap_page_range_noflush
> 1.29% [kernel] [k] alloc_pages_current
> 1.28% [kernel] [k] __free_vmap_area
> 1.25% [kernel] [k] prep_new_page
> 0.91% [kernel] [k] llist_add_batch
> 0.87% [kernel] [k] free_vmap_area_noflush
> 0.79% [kernel] [k] __vunmap
> 0.67% [kernel] [k] free_unref_page_prepare.part.69
> 0.60% [kernel] [k] _cond_resched
> <snip>
>
> See below some proposals:
>
> 1) we can maintain the pointer to last area we allocate from to have possibility
> of O(1) access to the block if permissive parameters allow that. Something
> like this:
>
> <snip>
> if (last_free_area.va && vstart == last_free_area.vstart &&
> align >= last_free_area.align &&
> size >= last_free_area.size)
>
> /* Use last cached node and do not lookup from the root of the tree */
> <snip>
>
> 2) Get rid of "busy" tree that stores allocated spaces or replaced it by something
> faster. We need it only for mapping va->va_start to vmap_area object when we release it.
>
> 3) We can remove vmap_area node from busy tree as soon as an object gets released.
> It becomes possible now, because we allocate from another tree. It will improve
> insertion time into "busy tree", otherwise it stays there until "lazy" logic removes
> it:
>
> <snip>
> @@ -1754,8 +1754,12 @@ void vm_unmap_ram(const void *mem, unsigned int count)
> return;
> }
>
> - va = find_vmap_area(addr);
> + spin_lock(&vmap_area_lock);
> + va = __find_vmap_area(addr);
> BUG_ON(!va);
> + unlink_va(va, &vmap_area_root);
> + spin_unlock(&vmap_area_lock);
> +
> debug_check_no_locks_freed((void *)va->va_start,
> (va->va_end - va->va_start));
> free_unmap_vmap_area(va);
> @@ -2162,6 +2166,7 @@ struct vm_struct *remove_vm_area(const void *addr)
> va->vm = NULL;
> va->flags &= ~VM_VM_AREA;
> va->flags |= VM_LAZY_FREE;
> + unlink_va(va, &vmap_area_root);
> spin_unlock(&vmap_area_lock);
>
> kasan_free_shadow(vm);
> <snip>
>
> All those things could be discussed over lkml. If there are some higher priority
> topics to discuss i do not want to waste the time and we can drop my proposal topic
> on the Kernel Summit.
>
I have missed at least one thing i wanted to mention. That is to make it
per CPU allocation. I mean to split the KVA space for NR_CPUS chunks and
make it lockless(or almost lockless). Probably it is possible to do that
only for specific zone, for example vmalloc space only. Lazelly drain
logic also цшдд be per-CPU.
But that is theory and probably looks like over optimization or so :)
--
Vlad Rezki
prev parent reply other threads:[~2019-06-02 18:31 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-30 6:05 [Ksummit-discuss] [TECH TOPIC] Reworking of KVA allocator in Linux kernel Theodore Ts'o
2019-05-31 2:51 ` Matthew Wilcox
2019-06-02 12:05 ` Uladzislau Rezki
2019-06-02 18:31 ` Uladzislau Rezki [this message]
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