x86/mm/tlb: Avoid reading mm_tlb_gen when possible

On extreme TLB shootdown storms, the mm's tlb_gen cacheline is highly
contended and reading it should (arguably) be avoided as much as
possible.

Currently, flush_tlb_func() reads the mm's tlb_gen unconditionally,
even when it is not necessary (e.g., the mm was already switched).
This is wasteful.

Moreover, one of the existing optimizations is to read mm's tlb_gen to
see if there are additional in-flight TLB invalidations and flush the
entire TLB in such a case. However, if the request's tlb_gen was already
flushed, the benefit of checking the mm's tlb_gen is likely to be offset
by the overhead of the check itself.

Running will-it-scale with tlb_flush1_threads show a considerable
benefit on 56-core Skylake (up to +24%):

threads		Baseline (v5.17+)	+Patch
1		159960			160202
5		310808			308378 (-0.7%)
10		479110			490728
15		526771			562528
20		534495			587316
25		547462			628296
30		579616			666313
35		594134			701814
40		612288			732967
45		617517			749727
50		637476			735497
55		614363			778913 (+24%)
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20220606180123.2485171-1-namit@vmware.com

arch/x86/mm/tlb.c

@@ -734,10 +734,10 @@ static void flush_tlb_func(void *info)
 	const struct flush_tlb_info *f = info;
 	struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);
 	u32 loaded_mm_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
-	u64 mm_tlb_gen = atomic64_read(&loaded_mm->context.tlb_gen);
 	u64 local_tlb_gen = this_cpu_read(cpu_tlbstate.ctxs[loaded_mm_asid].tlb_gen);
 	bool local = smp_processor_id() == f->initiating_cpu;
 	unsigned long nr_invalidate = 0;
+	u64 mm_tlb_gen;
 
 	/* This code cannot presently handle being reentered. */
 	VM_WARN_ON(!irqs_disabled());
@@ -771,6 +771,22 @@ static void flush_tlb_func(void *info)
 		return;
 	}
 
+	if (f->new_tlb_gen <= local_tlb_gen) {
+		/*
+		 * The TLB is already up to date in respect to f->new_tlb_gen.
+		 * While the core might be still behind mm_tlb_gen, checking
+		 * mm_tlb_gen unnecessarily would have negative caching effects
+		 * so avoid it.
+		 */
+		return;
+	}
+
+	/*
+	 * Defer mm_tlb_gen reading as long as possible to avoid cache
+	 * contention.
+	 */
+	mm_tlb_gen = atomic64_read(&loaded_mm->context.tlb_gen);
+
 	if (unlikely(local_tlb_gen == mm_tlb_gen)) {
 		/*
 		 * There's nothing to do: we're already up to date.  This can