Re: [PATCH] mm: Expose lazy vfree pages to control via sysctl

[Ashish Mhetre <amhetre@nvidia.com>]

The issue is not seen on new kernel. This patch won't be needed. Thanks.

On 06/01/19 2:12 PM, Ashish Mhetre wrote:
> Matthew, this issue was last reported in September 2018 on K4.9.
> I verified that the optimization patches mentioned by you were not 
> present in our downstream kernel when we faced the issue. I will check 
> whether issue still persist on new kernel with all these patches and 
> come back.
> 
> On 04/01/19 11:33 PM, Matthew Wilcox wrote:
>> On Fri, Jan 04, 2019 at 09:05:41PM +0530, Ashish Mhetre wrote:
>>> From: Hiroshi Doyu <hdoyu@nvidia.com>
>>>
>>> The purpose of lazy_max_pages is to gather virtual address space till it
>>> reaches the lazy_max_pages limit and then purge with a TLB flush and 
>>> hence
>>> reduce the number of global TLB flushes.
>>> The default value of lazy_max_pages with one CPU is 32MB and with 4 
>>> CPUs it
>>> is 96MB i.e. for 4 cores, 96MB of vmalloc space will be gathered 
>>> before it
>>> is purged with a TLB flush.
>>> This feature has shown random latency issues. For example, we have seen
>>> that the kernel thread for some camera application spent 30ms in
>>> __purge_vmap_area_lazy() with 4 CPUs.
>>
>> You're not the first to report something like this.  Looking through the
>> kernel logs, I see:
>>
>> commit 763b218ddfaf56761c19923beb7e16656f66ec62
>> Author: Joel Fernandes <joelaf@google.com>
>> Date:   Mon Dec 12 16:44:26 2016 -0800
>>
>>      mm: add preempt points into __purge_vmap_area_lazy()
>>
>> commit f9e09977671b618aeb25ddc0d4c9a84d5b5cde9d
>> Author: Christoph Hellwig <hch@lst.de>
>> Date:   Mon Dec 12 16:44:23 2016 -0800
>>
>>      mm: turn vmap_purge_lock into a mutex
>>
>> commit 80c4bd7a5e4368b680e0aeb57050a1b06eb573d8
>> Author: Chris Wilson <chris@chris-wilson.co.uk>
>> Date:   Fri May 20 16:57:38 2016 -0700
>>
>>      mm/vmalloc: keep a separate lazy-free list
>>
>> So the first thing I want to do is to confirm that you see this problem
>> on a modern kernel.  We've had trouble with NVidia before reporting
>> historical problems as if they were new.
>>