linux-kernel - Re: [RFC 00/14] Dynamic Kernel Stacks

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Message-ID: <f949f712-eacf-49a0-91ea-8062e2d1f5e0@zytor.com>
Date: Tue, 12 Mar 2024 14:36:27 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [RFC 00/14] Dynamic Kernel Stacks

On 3/12/24 12:45, Pasha Tatashin wrote:
>>
>> Ok, first of all, talking about "kernel memory" here is misleading.
> 
> Hi Peter,
> 
> I re-read my cover letter, and I do not see where "kernel memory" is
> mentioned. We are talking about kernel stacks overhead that is
> proportional to the user workload, as every active thread has an
> associated kernel stack. The idea is to save memory by not
> pre-allocating all pages of kernel-stacks, but instead use it as a
> safeguard when a stack actually becomes deep. Come-up with a solution
> that can handle rare deeper stacks only when needed. This could be
> done through faulting on the supported hardware (as proposed in this
> series), or via pre-map on every schedule event, and checking the
> access when thread goes off cpu (as proposed by Andy Lutomirski to
> avoid double faults on x86) .
> 
> In other words, this feature is only about one very specific type of
> kernel memory that is not even directly mapped (the feature required
> vmapped stacks).
> 
>> Unless your threads are spending nearly all their time sleeping, the
>> threads will occupy stack and TLS memory in user space as well.
> 
> Can you please elaborate, what data is contained in the kernel stack
> when thread is in user space? My series requires thread_info not to be
> in the stack by depending on THREAD_INFO_IN_TASK.
> 

My point is that what matters is total memory use, not just memory used 
in the kernel. Amdahl's law.

	-hpa