On Wed, May 14, 2014 at 07:17:29PM +0100, Ard Biesheuvel wrote:
> The Crypto Extensions based SHA1 implementation uses the NEON register file,
> and hence runs with preemption disabled. This patch adds a TIF_NEED_RESCHED
> check to its inner loop so we at least give up the CPU voluntarily when we
> are running in process context and have been tagged for preemption by the
> scheduler.
Sorry, I haven't got to the bottom of your series earlier and I now
realised that the last patches are not just new crypto algorithms.
> +static u8 const *sha1_do_update(struct shash_desc *desc, const u8 *data,
> + int blocks, u8 *head, unsigned int len)
> +{
> + struct sha1_state *sctx = shash_desc_ctx(desc);
> + struct thread_info *ti = NULL;
> +
> + /*
> + * Pass current's thread info pointer to sha1_ce_transform()
> + * below if we want it to play nice under preemption.
> + */
> + if ((IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY) || IS_ENABLED(CONFIG_PREEMPT))
> + && (desc->flags & CRYPTO_TFM_REQ_MAY_SLEEP))
> + ti = current_thread_info();
> +
> + do {
> + int rem;
> +
> + kernel_neon_begin_partial(16);
> + rem = sha1_ce_transform(blocks, data, sctx->state, head, 0, ti);
> + kernel_neon_end();
> +
> + data += (blocks - rem) * SHA1_BLOCK_SIZE;
> + blocks = rem;
> + head = NULL;
> + } while (unlikely(ti && blocks > 0));
> + return data;
> +}
What latencies are we talking about? Would it make sense to always
call cond_resched() even if preemption is disabled?
With PREEMPT_VOLUNTARY I don't think the above code does any voluntary
preemption. The preempt_enable() in kernel_neon_end() only reschedules
if PREEMPT.
But I think we should have this loop always rescheduling if
CRYPTO_TFM_REQ_MAY_SLEEP. I can see there is a crypto_yield() function
that conditionally reschedules. What's the overhead of calling
sha1_ce_transform() in a loop vs a single call for the entire data?
--
Catalin
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