Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Jun 13, 2022 at 02:55:47PM +0800, zhangfei.gao@foxmail.com wrote:
> Hi, Paul
> 
> On 2022/6/13 下午12:16, Paul E. McKenney wrote:
> > On Sun, Jun 12, 2022 at 08:57:11PM -0700, Paul E. McKenney wrote:
> > > On Mon, Jun 13, 2022 at 11:04:39AM +0800, zhangfei.gao@foxmail.com wrote:
> > > > Hi, Paul
> > > > 
> > > > On 2022/6/13 上午2:49, Paul E. McKenney wrote:
> > > > > On Sun, Jun 12, 2022 at 07:29:30PM +0200, Paolo Bonzini wrote:
> > > > > > On 6/12/22 18:40, Paul E. McKenney wrote:
> > > > > > > > Do these reserved memory regions really need to be allocated separately?
> > > > > > > > (For example, are they really all non-contiguous?  If not, that is, if
> > > > > > > > there are a lot of contiguous memory regions, could you sort the IORT
> > > > > > > > by address and do one ioctl() for each set of contiguous memory regions?)
> > > > > > > > 
> > > > > > > > Are all of these reserved memory regions set up before init is spawned?
> > > > > > > > 
> > > > > > > > Are all of these reserved memory regions set up while there is only a
> > > > > > > > single vCPU up and running?
> > > > > > > > 
> > > > > > > > Is the SRCU grace period really needed in this case?  (I freely confess
> > > > > > > > to not being all that familiar with KVM.)
> > > > > > > Oh, and there was a similar many-requests problem with networking many
> > > > > > > years ago.  This was solved by adding a new syscall/ioctl()/whatever
> > > > > > > that permitted many requests to be presented to the kernel with a single
> > > > > > > system call.
> > > > > > > 
> > > > > > > Could a new ioctl() be introduced that requested a large number
> > > > > > > of these memory regions in one go so as to make each call to
> > > > > > > synchronize_rcu_expedited() cover a useful fraction of your 9000+
> > > > > > > requests?  Adding a few of the KVM guys on CC for their thoughts.
> > > > > > Unfortunately not.  Apart from this specific case, in general the calls to
> > > > > > KVM_SET_USER_MEMORY_REGION are triggered by writes to I/O registers in the
> > > > > > guest, and those writes then map to a ioctl.  Typically the guest sets up a
> > > > > > device at a time, and each setup step causes a synchronize_srcu()---and
> > > > > > expedited at that.
> > > > > I was afraid of something like that...
> > > > > 
> > > > > > KVM has two SRCUs:
> > > > > > 
> > > > > > 1) kvm->irq_srcu is hardly relying on the "sleepable" part; it has readers
> > > > > > that are very very small, but it needs extremely fast detection of grace
> > > > > > periods; see commit 719d93cd5f5c ("kvm/irqchip: Speed up
> > > > > > KVM_SET_GSI_ROUTING", 2014-05-05) which split it off kvm->srcu.  Readers are
> > > > > > not so frequent.
> > > > > > 
> > > > > > 2) kvm->srcu is nastier because there are readers all the time.  The
> > > > > > read-side critical section are still short-ish, but they need the sleepable
> > > > > > part because they access user memory.
> > > > > Which one of these two is in play in this case?
> > > > > 
> > > > > > Writers are not frequent per se; the problem is they come in very large
> > > > > > bursts when a guest boots.  And while the whole boot path overall can be
> > > > > > quadratic, O(n) expensive calls to synchronize_srcu() can have a larger
> > > > > > impact on runtime than the O(n^2) parts, as demonstrated here.
> > > > > > 
> > > > > > Therefore, we operated on the assumption that the callers of
> > > > > > synchronized_srcu_expedited were _anyway_ busy running CPU-bound guest code
> > > > > > and the desire was to get past the booting phase as fast as possible.  If
> > > > > > the guest wants to eat host CPU it can "for(;;)" as much as it wants;
> > > > > > therefore, as long as expedited GPs didn't eat CPU *throughout the whole
> > > > > > system*, a preemptable busy wait in synchronize_srcu_expedited() were not
> > > > > > problematic.
> > > > > > 
> > > > > > This assumptions did match the SRCU code when kvm->srcu and kvm->irq_srcu
> > > > > > were was introduced (respectively in 2009 and 2014).  But perhaps they do
> > > > > > not hold anymore now that each SRCU is not as independent as it used to be
> > > > > > in those years, and instead they use workqueues instead?
> > > > > The problem was not internal to SRCU, but rather due to the fact
> > > > > that kernel live patching (KLP) had problems with the CPU-bound tasks
> > > > > resulting from repeated synchronize_rcu_expedited() invocations.  So I
> > > > > added heuristics to get the occasional sleep in there for KLP's benefit.
> > > > > Perhaps these heuristics need to be less aggressive about adding sleep.
> > > > > 
> > > > > These heuristics have these aspects:
> > > > > 
> > > > > 1.	The longer readers persist in an expedited SRCU grace period,
> > > > > 	the longer the wait between successive checks of the reader
> > > > > 	state.  Roughly speaking, we wait as long as the grace period
> > > > > 	has currently been in effect, capped at ten jiffies.
> > > > > 
> > > > > 2.	SRCU grace periods have several phases.  We reset so that each
> > > > > 	phase starts by not waiting (new phase, new set of readers,
> > > > > 	so don't penalize this set for the sins of the previous set).
> > > > > 	But once we get to the point of adding delay, we add the
> > > > > 	delay based on the beginning of the full grace period.
> > > > > 
> > > > > Right now, the checking for grace-period length does not allow for the
> > > > > possibility that a grace period might start just before the jiffies
> > > > > counter gets incremented (because I didn't realize that anyone cared),
> > > > > so that is one possible thing to change.  I can also allow more no-delay
> > > > > checks per SRCU grace-period phase.
> > > > > 
> > > > > Zhangfei, does something like the patch shown below help?
> > > > > 
> > > > > Additional adjustments are likely needed to avoid re-breaking KLP,
> > > > > but we have to start somewhere...
> > > > > 
> > > > > 							Thanx, Paul
> > > > > 
> > > > > ------------------------------------------------------------------------
> > > > > 
> > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > > > index 50ba70f019dea..6a354368ac1d1 100644
> > > > > --- a/kernel/rcu/srcutree.c
> > > > > +++ b/kernel/rcu/srcutree.c
> > > > > @@ -513,7 +513,7 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
> > > > >    #define SRCU_INTERVAL		1	// Base delay if no expedited GPs pending.
> > > > >    #define SRCU_MAX_INTERVAL	10	// Maximum incremental delay from slow readers.
> > > > > -#define SRCU_MAX_NODELAY_PHASE	1	// Maximum per-GP-phase consecutive no-delay instances.
> > > > > +#define SRCU_MAX_NODELAY_PHASE	3	// Maximum per-GP-phase consecutive no-delay instances.
> > > > >    #define SRCU_MAX_NODELAY	100	// Maximum consecutive no-delay instances.
> > > > >    /*
> > > > > @@ -522,12 +522,18 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
> > > > >     */
> > > > >    static unsigned long srcu_get_delay(struct srcu_struct *ssp)
> > > > >    {
> > > > > +	unsigned long gpstart;
> > > > > +	unsigned long j;
> > > > >    	unsigned long jbase = SRCU_INTERVAL;
> > > > >    	if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq), READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
> > > > >    		jbase = 0;
> > > > > -	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)))
> > > > > -		jbase += jiffies - READ_ONCE(ssp->srcu_gp_start);
> > > > > +	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq))) {
> > > > > +		j = jiffies - 1;
> > > > > +		gpstart = READ_ONCE(ssp->srcu_gp_start);
> > > > > +		if (time_after(j, gpstart))
> > > > > +			jbase += j - gpstart;
> > > > > +	}
> > > > >    	if (!jbase) {
> > > > >    		WRITE_ONCE(ssp->srcu_n_exp_nodelay, READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
> > > > >    		if (READ_ONCE(ssp->srcu_n_exp_nodelay) > SRCU_MAX_NODELAY_PHASE)
> > > > Unfortunately, this patch does not helpful.
> > > > 
> > > > Then re-add the debug info.
> > > > 
> > > > During the qemu boot
> > > > [  232.997667]  __synchronize_srcu loop=1000
> > > > 
> > > > [  361.094493]  __synchronize_srcu loop=9000
> > > > [  361.094501] Call trace:
> > > > [  361.094502]  dump_backtrace+0xe4/0xf0
> > > > [  361.094505]  show_stack+0x20/0x70
> > > > [  361.094507]  dump_stack_lvl+0x8c/0xb8
> > > > [  361.094509]  dump_stack+0x18/0x34
> > > > [  361.094511]  __synchronize_srcu+0x120/0x128
> > > > [  361.094514]  synchronize_srcu_expedited+0x2c/0x40
> > > > [  361.094515]  kvm_swap_active_memslots+0x130/0x198
> > > > [  361.094519]  kvm_activate_memslot+0x40/0x68
> > > > [  361.094520]  kvm_set_memslot+0x2f8/0x3b0
> > > > [  361.094523]  __kvm_set_memory_region+0x2e4/0x438
> > > > [  361.094524]  kvm_set_memory_region+0x78/0xb8
> > > > [  361.094526]  kvm_vm_ioctl+0x5a0/0x13e0
> > > > [  361.094528]  __arm64_sys_ioctl+0xb0/0xf8
> > > > [  361.094530]  invoke_syscall+0x4c/0x110
> > > > [  361.094533]  el0_svc_common.constprop.0+0x68/0x128
> > > > [  361.094536]  do_el0_svc+0x34/0xc0
> > > > [  361.094538]  el0_svc+0x30/0x98
> > > > [  361.094541]  el0t_64_sync_handler+0xb8/0xc0
> > > > [  361.094544]  el0t_64_sync+0x18c/0x190
> > > > [  363.942817]  kvm_set_memory_region loop=6000
> > > Huh.
> > > 
> > > One possibility is that the "if (!jbase)" block needs to be nested
> > > within the "if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq))) {" block.
> 
> I test this diff and NO helpful
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 50ba70f019de..36286a4b74e6 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -513,7 +513,7 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
> 
>  #define SRCU_INTERVAL          1       // Base delay if no expedited GPs
> pending.
>  #define SRCU_MAX_INTERVAL      10      // Maximum incremental delay from
> slow readers.
> -#define SRCU_MAX_NODELAY_PHASE 1       // Maximum per-GP-phase consecutive
> no-delay instances.
> +#define SRCU_MAX_NODELAY_PHASE 3       // Maximum per-GP-phase consecutive
> no-delay instances.
>  #define SRCU_MAX_NODELAY       100     // Maximum consecutive no-delay
> instances.
> 
>  /*
> @@ -522,16 +522,23 @@ static bool srcu_readers_active(struct srcu_struct
> *ssp)
>   */
>  static unsigned long srcu_get_delay(struct srcu_struct *ssp)
>  {
> +       unsigned long gpstart;
> +       unsigned long j;
>         unsigned long jbase = SRCU_INTERVAL;
> 
>         if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq),
> READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
>                 jbase = 0;
> -       if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)))
> -               jbase += jiffies - READ_ONCE(ssp->srcu_gp_start);
> -       if (!jbase) {
> -               WRITE_ONCE(ssp->srcu_n_exp_nodelay,
> READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
> -               if (READ_ONCE(ssp->srcu_n_exp_nodelay) >
> SRCU_MAX_NODELAY_PHASE)
> -                       jbase = 1;
> +       if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq))) {
> +               j = jiffies - 1;
> +               gpstart = READ_ONCE(ssp->srcu_gp_start);
> +               if (time_after(j, gpstart))
> +                       jbase += j - gpstart;
> +
> +               if (!jbase) {
> +                       WRITE_ONCE(ssp->srcu_n_exp_nodelay,
> READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
> +                       if (READ_ONCE(ssp->srcu_n_exp_nodelay) >
> SRCU_MAX_NODELAY_PHASE)
> +                               jbase = 1;
> +               }
>         }

That is in fact what I was intending you to test, thank you.  As you
say, unfortunately it did not help.

Could you please test removing the "if (!jbase)" block entirely?

> > And when I run 10,000 consecutive synchronize_rcu_expedited() calls, the
> > above change reduces the overhead by more than an order of magnitude.
> > Except that the overhead of the series is far less than one second,
> > not the several minutes that you are seeing.  So the per-call overhead
> > decreases from about 17 microseconds to a bit more than one microsecond.
> > 
> > I could imagine an extra order of magnitude if you are running HZ=100
> > instead of the HZ=1000 that I am running.  But that only gets up to a
> > few seconds.

One possible reason for the difference would be if your code has
SRCU readers.

Could you please tell me the value of CONFIG_HZ on your system?
Also the value of CONFIG_PREEMPTION?

> > > One additional debug is to apply the patch below on top of the one you
> apply the patch below?
> > > just now kindly tested, then use whatever debug technique you wish to
> > > work out what fraction of the time during that critical interval that
> > > srcu_get_delay() returns non-zero.
> Sorry, I am confused, no patch right?

Apologies, my omission.

> Just measure srcu_get_delay return to non-zero?

Exactly, please!

> By the way, the issue should be only related with qemu apci. not related
> with rmr feature
> Test with: https://github.com/qemu/qemu/tree/stable-6.1
> 
> Looks it caused by too many kvm_region_add & kvm_region_del if acpi=force,
> If no acpi, no print kvm_region_add/del (1000 times print once)
> 
> If with acpi=force,
> During qemu boot
> kvm_region_add region_add = 1000
> kvm_region_del region_del = 1000
> kvm_region_add region_add = 2000
> kvm_region_del region_del = 2000
> kvm_region_add region_add = 3000
> kvm_region_del region_del = 3000
> kvm_region_add region_add = 4000
> kvm_region_del region_del = 4000
> kvm_region_add region_add = 5000
> kvm_region_del region_del = 5000
> kvm_region_add region_add = 6000
> kvm_region_del region_del = 6000
> 
> kvm_region_add/kvm_region_del ->
> kvm_set_phys_mem->
> kvm_set_user_memory_region->
> kvm_vm_ioctl(s, KVM_SET_USER_MEMORY_REGION, &mem)
> 
> [  361.094493]  __synchronize_srcu loop=9000
> [  361.094501] Call trace:
> [  361.094502]  dump_backtrace+0xe4/0xf0
> [  361.094505]  show_stack+0x20/0x70
> [  361.094507]  dump_stack_lvl+0x8c/0xb8
> [  361.094509]  dump_stack+0x18/0x34
> [  361.094511]  __synchronize_srcu+0x120/0x128
> [  361.094514]  synchronize_srcu_expedited+0x2c/0x40
> [  361.094515]  kvm_swap_active_memslots+0x130/0x198
> [  361.094519]  kvm_activate_memslot+0x40/0x68
> [  361.094520]  kvm_set_memslot+0x2f8/0x3b0
> [  361.094523]  __kvm_set_memory_region+0x2e4/0x438
> [  361.094524]  kvm_set_memory_region+0x78/0xb8
> [  361.094526]  kvm_vm_ioctl+0x5a0/0x13e0
> [  361.094528]  __arm64_sys_ioctl+0xb0/0xf8
> [  361.094530]  invoke_syscall+0x4c/0x110
> [  361.094533]  el0_svc_common.constprop.0+0x68/0x128
> [  361.094536]  do_el0_svc+0x34/0xc0
> [  361.094538]  el0_svc+0x30/0x98
> [  361.094541]  el0t_64_sync_handler+0xb8/0xc0
> [  361.094544]  el0t_64_sync+0x18c/0x190
> [  363.942817]  kvm_set_memory_region loop=6000

Good to know, thank you!

							Thanx, Paul