linux-kernel - Re: [PATCH] x86/resctrl: avoid compiler optimization in __resctrl_sched

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Date:   Mon, 6 Mar 2023 13:01:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     linux-kernel@...r.kernel.org, tony.luck@...el.com,
        reinette.chatre@...el.com, fenghua.yu@...el.com,
        peternewman@...gle.com, bp@...e.de, james.morse@....com,
        babu.moger@....com, ananth.narayan@....com, vschneid@...hat.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH] x86/resctrl: avoid compiler optimization in
 __resctrl_sched_in

On Fri, Mar 03, 2023 at 03:11:33PM -0800, Stephane Eranian wrote:

> The problem is located in the __resctrl_sched_in() routine which rewrites
> the active closid via the PQR_ASSOC register. Because this is an expensive
> operation, the kernel only does it when the context switch involves tasks
> with different CLOSID. And to check that, it needs to access the current
> task's closid field using current->closid. current is actually a macro
> that reads the per-cpu variable pcpu_hot.current_task.
> 
> After an investigation by compiler experts, the problem has been tracked down
> to the usage of the get_current() macro in the __resctrl_sched_in() code and
> in particular the per-cpu macro:
> 
> static __always_inline struct task_struct *get_current(void)
> {
>         return this_cpu_read_stable(pcpu_hot.current_task);
> }
> 
> And as per percpu.h:
> 
> /*
>  * this_cpu_read() makes gcc load the percpu variable every time it is
>  * accessed while this_cpu_read_stable() allows the value to be cached.
>  * this_cpu_read_stable() is more efficient and can be used if its value
>  * is guaranteed to be valid across cpus.  The current users include
>  * get_current() and get_thread_info() both of which are actually
>  * per-thread variables implemented as per-cpu variables and thus
>  * stable for the duration of the respective task.
>  */
> 
> The _stable version of the macro allows the value to be cached, meaning it
> does not force a reload.

Right, so afaict the difference between this_cpu_read() and
this_cpu_read_stable() is the volatile qualifier.

this_cpu_read() is asm volatile(), while this_cpu_read_stable() and
raw_cpu_read() are both an unqualified asm().

Now, afaiu we're inlining all of this into __switch_to(), which has
raw_cpu_write(pcpu_hot.current_task, next_p).

And I suppose what the compiler is doing is lifting the 'current' load
over that store, but how is it allowed that? I thought C was supposed to
have PO consistency, That raw_cpu_write() should be seen as a store to
to pcpu_hot.current_task, why can it lift a load over the store?

Specifically, percpu_to_op() has a "+m" output constaint while
percpu_stable_op() has a "p" input constraint on the same address.

Compiler folks help?