Re: [PATCH] x86/resctrl: avoid compiler optimization in __resctrl_sched_in

[Peter Zijlstra]

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?