linux-kernel - Re: [PATCH v4 10/24] crypto: x86/poly

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Date:   Fri, 25 Nov 2022 16:40:50 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Robert Elliott <elliott@....com>, davem@...emloft.net,
        tim.c.chen@...ux.intel.com, ap420073@...il.com, ardb@...nel.org,
        David.Laight@...lab.com, ebiggers@...nel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 10/24] crypto: x86/poly - limit FPU preemption

On Wed, Nov 16, 2022 at 12:13:51PM +0100, Jason A. Donenfeld wrote:
> On Tue, Nov 15, 2022 at 10:13:28PM -0600, Robert Elliott wrote:
> > +/* avoid kernel_fpu_begin/end scheduler/rcu stalls */
> > +static const unsigned int bytes_per_fpu = 337 * 1024;
> > +
> 
> Use an enum for constants like this:
> 
>     enum { BYTES_PER_FPU = ... };
> 
> You can even make it function-local, so it's near the code that uses it,
> which will better justify its existence.
> 
> Also, where did you get this number? Seems kind of weird.

These numbers are highly dependent on hardware and I think having
them hard-coded is wrong.

Perhaps we should try a different approach.  How about just limiting
the size to 4K, and then depending on need_resched we break out of
the loop? Something like:

	if (!len)
		return 0;

	kernel_fpu_begin();
	for (;;) {
		unsigned int chunk = min(len, 4096);

		sha1_base_do_update(desc, data, chunk, sha1_xform);

		len -= chunk;
		data += chunk;

		if (!len)
			break;

		if (need_resched()) {
			kernel_fpu_end();
			cond_resched();
			kernel_fpu_begin();
		}
	}
	kernel_fpu_end();

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt