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

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Date: Mon, 18 Mar 2024 13:00:52 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: David Laight <David.Laight@...lab.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Matthew Wilcox <willy@...radead.org>, 
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Subject: Re: [RFC 00/14] Dynamic Kernel Stacks

On Mon, Mar 18, 2024 at 11:39 AM David Laight <David.Laight@...lab.com> wrote:
>
> ...
> > - exit_to_user_mode(): Unmap the extra three pages and return them to
> > the per-CPU cache. This function is called late in the kernel exit
> > path.
>
> Why bother?
> The number of tasks running in user_mode is limited to the number
> of cpu. So the most you save is a few pages per cpu.
>
> Plausibly a context switch from an interrupt (eg timer tick)
> could suspend a task without saving anything on its kernel stack.
> But how common is that in reality?
> In a well behaved system most user threads will be sleeping on
> some event - so with an active kernel stack.
>
> I can also imagine that something like sys_epoll() actually
> sleeps with not (that much) stack allocated.
> But the calls into all the drivers to check the status
> could easily go into another page.
> You really wouldn't to keep allocating and deallocating
> physical pages (which I'm sure has TLB flushing costs)
> all the time for those processes.
>
> Perhaps a 'garbage collection' activity that reclaims stack
> pages from processes that have been asleep 'for a while' or
> haven't used a lot of stack recently (if hw 'page accessed'
> bit can be used) might make more sense.
>
> Have you done any instrumentation to see which system calls
> are actually using more than (say) 8k of stack?
> And how often the user threads that make those calls do so?

None of our syscalls, AFAIK.

Pasha

>
>         David
>
> -
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