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Date:   Thu, 5 Oct 2023 22:42:17 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     "Chen, Tim C" <tim.c.chen@intel.com>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Chinner <dchinner@redhat.com>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Carlos Maiolino <cem@kernel.org>,
        Chuck Lever <chuck.lever@oracle.com>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: RE: [PATCH 8/8] shmem,percpu_counter: add _limited_add(fbc, limit,
 amount)
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On Thu, 5 Oct 2023, Chen, Tim C wrote:

> >--- a/lib/percpu_counter.c
> >+++ b/lib/percpu_counter.c
> >@@ -278,6 +278,59 @@ int __percpu_counter_compare(struct
> >percpu_counter *fbc, s64 rhs, s32 batch)  }
> >EXPORT_SYMBOL(__percpu_counter_compare);
> >
> >+/*
> >+ * Compare counter, and add amount if the total is within limit.
> >+ * Return true if amount was added, false if it would exceed limit.
> >+ */
> >+bool __percpu_counter_limited_add(struct percpu_counter *fbc,
> >+				  s64 limit, s64 amount, s32 batch) {
> >+	s64 count;
> >+	s64 unknown;
> >+	unsigned long flags;
> >+	bool good;
> >+
> >+	if (amount > limit)
> >+		return false;
> >+
> >+	local_irq_save(flags);
> >+	unknown = batch * num_online_cpus();
> >+	count = __this_cpu_read(*fbc->counters);
> >+
> >+	/* Skip taking the lock when safe */
> >+	if (abs(count + amount) <= batch &&
> >+	    fbc->count + unknown <= limit) {
> >+		this_cpu_add(*fbc->counters, amount);
> >+		local_irq_restore(flags);
> >+		return true;
> >+	}
> >+
> >+	raw_spin_lock(&fbc->lock);
> >+	count = fbc->count + amount;
> >+
> 
> Perhaps we can fast path the case where for sure
> we will exceed limit? 
> 
> if (fbc->count + amount - unknown > limit)
> 	return false;

Thanks, that sounds reasonable: I'll try to add something like that -
but haven't thought about it carefully enough yet (too easy for me
to overlook some negative case which messes everything up).

Hugh