Re: [RFC PATCH 00/14] context_tracking,x86: Defer some IPIs until a user->kernel transition

[Valentin Schneider]

On 05/07/23 18:48, Nadav Amit wrote:
>> On Jul 5, 2023, at 11:12 AM, Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
>>
>> Deferral approach
>> =================
>>
>> Storing each and every callback, like a secondary call_single_queue turned out
>> to be a no-go: the whole point of deferral is to keep NOHZ_FULL CPUs in
>> userspace for as long as possible - no signal of any form would be sent when
>> deferring an IPI. This means that any form of queuing for deferred callbacks
>> would end up as a convoluted memory leak.
>>
>> Deferred IPIs must thus be coalesced, which this series achieves by assigning
>> IPIs a "type" and having a mapping of IPI type to callback, leveraged upon
>> kernel entry.
>
> I have some experience with similar an optimization. Overall, it can make
> sense and as you show, it can reduce the number of interrupts.
>
> The main problem of such an approach might be in cases where a process
> frequently enters and exits the kernel between deferred-IPIs, or even worse -
> the IPI is sent while the remote CPU is inside the kernel. In such cases, you
> pay the extra cost of synchronization and cache traffic, and might not even
> get the benefit of reducing the number of IPIs.
>
> In a sense, it's a more extreme case of the overhead that x86’s lazy-TLB
> mechanism introduces while tracking whether a process is running or not. But
> lazy-TLB would change is_lazy much less frequently than context tracking,
> which means that the deferring the IPIs as done in this patch-set has a
> greater potential to hurt performance than lazy-TLB.
>
> tl;dr - it would be beneficial to show some performance number for both a
> “good” case where a process spends most of the time in userspace, and “bad”
> one where a process enters and exits the kernel very frequently. Reducing
> the number of IPIs is good but I don’t think it is a goal by its own.
>

There already is a significant overhead incurred on kernel entry for
nohz_full CPUs due to all of context_tracking faff; now I *am* making it
worse with that extra atomic, but I get the feeling it's not going to stay
:D

nohz_full CPUs that do context transitions very frequently are
unfortunately in the realm of "you shouldn't do that". Due to what's out
there I have to care about *occasional* transitions, but some folks
consider even that to be broken usage, so I don't believe getting numbers
for that to be much relevant.

> [ BTW: I did not go over the patches in detail. Obviously, there are
>   various delicate points that need to be checked, as avoiding the
>   deferring of IPIs if page-tables are freed. ]