linux-kernel - Re: [PATCH v3 04/11] mm: vmalloc: Remove global vmap_area

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Date: Sun,  7 Jan 2024 14:59:42 +0800
From: Hillf Danton <hdanton@...a.com>
To: Uladzislau Rezki <urezki@...il.com>,
	Wen Gu <guwen@...ux.alibaba.com>
Cc: linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 04/11] mm: vmalloc: Remove global vmap_area_root rb-tree

On Sat, 6 Jan 2024 17:36:23 +0100 Uladzislau Rezki <urezki@...il.com>
> > 
> > Thank you! I tried the patch, and it seems that the wait for rwlock_t
> > also exists, as much as using spinlock_t. (The flamegraph is attached.
> > Not sure why the read_lock waits so long, given that there is no frequent
> > write_lock competition)
> > 
> >                vzalloced shmem(spinlock_t)   vzalloced shmem(rwlock_t)
> > Requests/sec         583729.93                     460007.44
> > 
> > So I guess the overhead in finding vmap area is inevitable here and the
> > original spin_lock is fine in this series.
> > 
> I have also noticed a erformance difference between rwlock and spinlock. 
> So, yes. This is what we need to do extra if CONFIG_HARDENED_USERCOPY is
> set, i.e. find a VA.

See if read bias helps to understand the gap between spinlock and rwlock.

--- x/kernel/locking/qrwlock.c
+++ y/kernel/locking/qrwlock.c
@@ -23,7 +23,7 @@ void __lockfunc queued_read_lock_slowpat
 	/*
 	 * Readers come here when they cannot get the lock without waiting
 	 */
-	if (unlikely(in_interrupt())) {
+	if (1) {
 		/*
 		 * Readers in interrupt context will get the lock immediately
 		 * if the writer is just waiting (not holding the lock yet),