Subject: Re: Excessive TLB flush ranges

From: "Russell King (Oracle)" <[email-masked]>
Date: Mon, 15 May 2023 17:59:11 +0100

On Mon, May 15, 2023 at 06:43:40PM +0200, Thomas Gleixner wrote:
> Folks!
> 
> We're observing massive latencies and slowdowns on ARM32 machines due to
> excessive TLB flush ranges.
> 
> Those can be observed when tearing down a process, which has a seccomp
> BPF filter installed. ARM32 uses the vmalloc area for module space.
> 
> bpf_prog_free_deferred()
>   vfree()
>     _vm_unmap_aliases()
>        collect_per_cpu_vmap_blocks: start:0x95c8d000 end:0x95c8e000 size:0x1000 
>        __purge_vmap_area_lazy(start:0x95c8d000, end:0x95c8e000)
> 
>          va_start:0xf08a1000 va_end:0xf08a5000 size:0x00004000 gap:0x5ac13000 (371731 pages)
>          va_start:0xf08a5000 va_end:0xf08a9000 size:0x00004000 gap:0x00000000 (     0 pages)
>          va_start:0xf08a9000 va_end:0xf08ad000 size:0x00004000 gap:0x00000000 (     0 pages)
>          va_start:0xf08ad000 va_end:0xf08b1000 size:0x00004000 gap:0x00000000 (     0 pages)
>          va_start:0xf08b3000 va_end:0xf08b7000 size:0x00004000 gap:0x00002000 (     2 pages)
>          va_start:0xf08b7000 va_end:0xf08bb000 size:0x00004000 gap:0x00000000 (     0 pages)
>          va_start:0xf08bb000 va_end:0xf08bf000 size:0x00004000 gap:0x00000000 (     0 pages)
>          va_start:0xf0a15000 va_end:0xf0a17000 size:0x00002000 gap:0x00156000 (   342 pages)
> 
>       flush_tlb_kernel_range(start:0x95c8d000, end:0xf0a17000)
> 
>          Does 372106 flush operations where only 31 are useful

So, you asked the architecture to flush a large range, and are then
surprised if it takes a long time. There is no way to know how many
of those are useful.

Now, while using the sledge hammer of flushing all TLB entries may
sound like a good answer, if we're only evicting 31 entries, the
other entries are probably useful to have, no?

I think that you'd only run into this if you had a huge BPF
program and you tore it down, no?

-- 
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