Re: [PATCH] signal: Skip the altstack update when not needed

[Dave Hansen]

On 12/9/21 3:24 PM, Chang S. Bae wrote:
> New x86 FPU features require a large signal stack for their large states.
> Instead of requiring a large stack for every process, make sure enough
> altstack both at sys_sigaltstack() and when enabling the feature in each
> process.

This is jumping into the imperative voice when describing the
background.  It's rather jarring

> The optional size check was added. It helps to reject a too-small altstack
> when the large feature is enabled. Also, the architecture code examines
> each thread's altstack large enough before enabling the feature.
> 
> But threads can be racy without a lock. So, this enforcement mechanism
> accompanies a lock to serialize altstack updates and the size check.
> 
> On the signal return path, the altstack is restored via do_sigaltstack().
> In fact, the threads without altstack ensure it is disabled there. While no
> altstack change is needed in this case, this call ends up obtaining the
> lock.
> 
> When multiple signal returns hit the lock at the same time, this
> unnecessarily increases the lock contention.
> 
> Add a new check to avoid this. Check if an altstack update is needed. If
> not, skip the lock and the update. This may help sys_sigaltstack() in
> general. So place it in the function.

How about:

== Background ==

Support for large, "dynamic" fpstates was recently merged.  This
included code to ensure that sigaltstacks are sufficiently sized for
these large states.  A new lock was added to remove races between
enabling large features and setting up sigaltstacks.

== Problem ==

The new lock (sigaltstack_lock()) is acquired in the sigreturn path
before restoring the old sigaltstack.  Unfortunately, contention on the
new lock causes a measurable signal handling performance regression[link
here].  However, the common case is that no *changes* are made to the
sigaltstack state at sigreturn.

== Solution ==

do_sigaltstack() acquires sigaltstack_lock() and is used for both
sys_sigaltstack() and restoring the sigaltstack in sys_sigreturn().
Check for changes to the sigaltstack before taking the lock.  If no
changes were made, return before acquiring the lock.

This removes lock contention from the common-case sigreturn path.


> diff --git a/kernel/signal.c b/kernel/signal.c
> index a629b11bf3e0..eeb634f954cd 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -4185,6 +4185,11 @@ do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp,
>  				ss_mode != 0))
>  			return -EINVAL;
>  
> +		if (t->sas_ss_sp == (unsigned long)ss_sp &&
> +		    t->sas_ss_size == ss_size &&
> +		    t->sas_ss_flags == ss_flags)
> +			return 0;

This needs something like:

		/*
		 * Return before taking any locks if no actual
		 * sigaltstack changes were requested.
		 */

>  		sigaltstack_lock();
>  		if (ss_mode == SS_DISABLE) {
>  			ss_size = 0;
>