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Date: Thu, 12 Feb 2009 11:16:10 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...e.hu>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
Thomas Gleixner <tglx@...x.de>,
LKML <linux-kernel@...r.kernel.org>,
rt-users <linux-rt-users@...r.kernel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Carsten Emde <ce@...g.ch>,
Clark Williams <williams@...hat.com>,
rusty <rusty@...tcorp.com.au>
Subject: Re: [patch] generic-ipi: remove kmalloc, cleanup
On Thu, 2009-02-12 at 11:07 +0100, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
>
> > On Thu, 2009-02-12 at 09:19 +0100, Ingo Molnar wrote:
> > > * Ingo Molnar <mingo@...e.hu> wrote:
> > >
> > > > hm, that's a complex one - we do kfree() from IPI context, [...]
> > >
> > > The patch below might do the trick - it offloads this to a softirq.
> > > Not tested yet.
> >
> > The simple fix is something like:
> >
> > ---
> > kernel/smp.c | 8 ++++++++
> > 1 files changed, 8 insertions(+), 0 deletions(-)
>
> ok, i made it unconditional (not just a PREEMPT_RT hac) and did the
> cleanup below on top of it.
>
> I dont think repeat, queued IPIs are all that interesting from a
> performance point of view. If they are, it will all be clearly
> bisectable.
Right, except I really don't like the smp_call_function_many() slow path
that's now the only path.
Rusty did that I think, but he also had some idea on how to fix it, I
think it boiled down to sticking a count in the call data instead of the
full cpumask.
So I'd rather we first fix that code, and then remove the kmalloc
all-together like you propose here.
> Ingo
>
> --------------->
> Subject: generic-ipi: remove kmalloc, cleanup
> From: Ingo Molnar <mingo@...e.hu>
>
> Now that we dont use the kmalloc() sequence anymore, remove
> CSD_FLAG_ALLOC and all its dependencies.
>
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> ---
> kernel/smp.c | 86 +++++++++++------------------------------------------------
> 1 file changed, 17 insertions(+), 69 deletions(-)
>
> Index: tip/kernel/smp.c
> ===================================================================
> --- tip.orig/kernel/smp.c
> +++ tip/kernel/smp.c
> @@ -17,8 +17,7 @@ __cacheline_aligned_in_smp DEFINE_RAW_SP
>
> enum {
> CSD_FLAG_WAIT = 0x01,
> - CSD_FLAG_ALLOC = 0x02,
> - CSD_FLAG_LOCK = 0x04,
> + CSD_FLAG_LOCK = 0x02,
> };
>
> struct call_function_data {
> @@ -85,15 +84,6 @@ static void generic_exec_single(int cpu,
> csd_flag_wait(data);
> }
>
> -static void rcu_free_call_data(struct rcu_head *head)
> -{
> - struct call_function_data *data;
> -
> - data = container_of(head, struct call_function_data, rcu_head);
> -
> - kfree(data);
> -}
> -
> /*
> * Invoked by arch to handle an IPI for call function. Must be called with
> * interrupts disabled.
> @@ -138,8 +128,6 @@ void generic_smp_call_function_interrupt
> smp_wmb();
> data->csd.flags &= ~CSD_FLAG_WAIT;
> }
> - if (data->csd.flags & CSD_FLAG_ALLOC)
> - call_rcu(&data->rcu_head, rcu_free_call_data);
> }
> rcu_read_unlock();
>
> @@ -190,8 +178,7 @@ void generic_smp_call_function_single_in
> } else if (data_flags & CSD_FLAG_LOCK) {
> smp_wmb();
> data->flags &= ~CSD_FLAG_LOCK;
> - } else if (data_flags & CSD_FLAG_ALLOC)
> - kfree(data);
> + }
> }
> /*
> * See comment on outer loop
> @@ -236,13 +223,11 @@ int smp_call_function_single(int cpu, vo
> /*
> * We are calling a function on a single CPU
> * and we are not going to wait for it to finish.
> - * We first try to allocate the data, but if we
> - * fail, we fall back to use a per cpu data to pass
> - * the information to that CPU. Since all callers
> - * of this code will use the same data, we must
> - * synchronize the callers to prevent a new caller
> - * from corrupting the data before the callee
> - * can access it.
> + * We use a per cpu data to pass the information to
> + * that CPU. Since all callers of this code will use
> + * the same data, we must synchronize the callers to
> + * prevent a new caller from corrupting the data before
> + * the callee can access it.
> *
> * The CSD_FLAG_LOCK is used to let us know when
> * the IPI handler is done with the data.
> @@ -252,15 +237,10 @@ int smp_call_function_single(int cpu, vo
> * will make sure the callee is done with the
> * data before a new caller will use it.
> */
> - data = NULL;
> - if (data)
> - data->flags = CSD_FLAG_ALLOC;
> - else {
> - data = &per_cpu(csd_data, me);
> - while (data->flags & CSD_FLAG_LOCK)
> - cpu_relax();
> - data->flags = CSD_FLAG_LOCK;
> - }
> + data = &per_cpu(csd_data, me);
> + while (data->flags & CSD_FLAG_LOCK)
> + cpu_relax();
> + data->flags = CSD_FLAG_LOCK;
> } else {
> data = &d;
> data->flags = CSD_FLAG_WAIT;
> @@ -321,8 +301,6 @@ void smp_call_function_many(const struct
> void (*func)(void *), void *info,
> bool wait)
> {
> - struct call_function_data *data;
> - unsigned long flags;
> int cpu, next_cpu;
>
> /* Can deadlock when called with interrupts disabled */
> @@ -347,43 +325,13 @@ void smp_call_function_many(const struct
> return;
> }
>
> - data = NULL;
> - if (unlikely(!data)) {
> - /* Slow path. */
> - for_each_online_cpu(cpu) {
> - if (cpu == smp_processor_id())
> - continue;
> - if (cpumask_test_cpu(cpu, mask))
> - smp_call_function_single(cpu, func, info, wait);
> - }
> - return;
> + /* Slow path. */
> + for_each_online_cpu(cpu) {
> + if (cpu == smp_processor_id())
> + continue;
> + if (cpumask_test_cpu(cpu, mask))
> + smp_call_function_single(cpu, func, info, wait);
> }
> -
> - spin_lock_init(&data->lock);
> - data->csd.flags = CSD_FLAG_ALLOC;
> - if (wait)
> - data->csd.flags |= CSD_FLAG_WAIT;
> - data->csd.func = func;
> - data->csd.info = info;
> - cpumask_and(to_cpumask(data->cpumask_bits), mask, cpu_online_mask);
> - cpumask_clear_cpu(smp_processor_id(), to_cpumask(data->cpumask_bits));
> - data->refs = cpumask_weight(to_cpumask(data->cpumask_bits));
> -
> - spin_lock_irqsave(&call_function_lock, flags);
> - list_add_tail_rcu(&data->csd.list, &call_function_queue);
> - spin_unlock_irqrestore(&call_function_lock, flags);
> -
> - /*
> - * Make the list addition visible before sending the ipi.
> - */
> - smp_mb();
> -
> - /* Send a message to all CPUs in the map */
> - arch_send_call_function_ipi_mask(to_cpumask(data->cpumask_bits));
> -
> - /* optionally wait for the CPUs to complete */
> - if (wait)
> - csd_flag_wait(&data->csd);
> }
> EXPORT_SYMBOL(smp_call_function_many);
>
--
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