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    SubjectRe: [RFC][RT][PATCH] mm: Do not use stop_machine() for __zone_pcp_udpate() for CONFIG_PREEMPT_RT_FULL
    Adding to the distribution list: the author of the patch that included
    the call to stop_machine() (commit 112067f0905b2de862c607ee62411cf47d2fe5c4).

    On 06/19/12 11:32, Aaditya Kumar wrote:
    > The code path of __zone_pcp_update() has following locks, which in
    > CONFIG_PREEMPT_RT_FULL=y are rt-mutex.
    > - pa_lock locked by cpu_lock_irqsave()
    > - zone->lock locked by free_pcppages_bulk()
    >
    > Since __zone_pcp_update() is called from stop_machine(), so with
    > CONFIG_PREEMPT_RT_FULL=y
    > we get following backtrace when __zone_pcp_update() is called during
    > memory hot plugging while
    > doing heavy file I/O.
    >
    > I think stop_machine() may not be required for calling __zone_pcp_update()
    > in case of CONFIG_PREEMPT_RT_FULL=y as acquiring pa_lock in __zone_pcp_update()
    > should be sufficient to isolate pcp pages and to setup per cpu pagesets.
    >
    > Can someone please let me know if am missing anything here?
    >
    >
    > The backtrace that this patch fixes:
    > BUG: scheduling while atomic: migration/0/7/0x00000002
    > Modules linked in: v2p
    > Backtrace:
    > [<800111a0>] (dump_backtrace+0x0/0x10c) from [<802d7b7c>]
    > (dump_stack+0x18/0x1c)
    > r6:80c8fc28 r5:80c8f9a0 r4:00000000 r3:60000013
    > [<802d7b64>] (dump_stack+0x0/0x1c) from [<8001e81c>] (__schedule_bug+0x64/0x74)
    > [<8001e7b8>] (__schedule_bug+0x0/0x74) from [<802d7fa0>]
    > (__schedule+0x68/0x604)
    > r4:8051bf00 r3:00000000
    > [<802d7f38>] (__schedule+0x0/0x604) from [<802d8a78>] (schedule+0x98/0xbc)
    > [<802d89e0>] (schedule+0x0/0xbc) from [<802d9e14>]
    > (rt_spin_lock_slowlock+0x168/0x240)
    > r4:805228f4 r3:00000000
    > [<802d9cac>] (rt_spin_lock_slowlock+0x0/0x240) from [<802da234>]
    > (rt_spin_lock+0x10/0x14)
    > [<802da224>] (rt_spin_lock+0x0/0x14) from [<8008694c>]
    > (__zone_pcp_update+0x58/0xd8)
    > [<800868f4>] (__zone_pcp_update+0x0/0xd8) from [<800603ec>]
    > (stop_machine_cpu_stop+0xb0/0x104)
    > [<8006033c>] (stop_machine_cpu_stop+0x0/0x104) from [<80060200>]
    > (cpu_stopper_thread+0xd4/0x188)
    >
    >
    > Signed-off-by: Aaditya Kumar <aaditya.kumar@ap.sony.com>
    >
    > ---
    > mm/page_alloc.c | 4 4 + 0 - 0 !
    > 1 file changed, 4 insertions(+)
    >
    > Index: b/mm/page_alloc.c
    > ===================================================================
    > --- a/mm/page_alloc.c
    > +++ b/mm/page_alloc.c
    > @@ -3868,7 +3868,11 @@ static int __zone_pcp_update(void *data)
    >
    > void zone_pcp_update(struct zone *zone)
    > {
    > +#ifndef CONFIG_PREEMPT_RT_FULL
    > stop_machine(__zone_pcp_update, zone, NULL);
    > +#else
    > + __zone_pcp_update(zone);
    > +#endif
    > }
    >
    > static __meminit void zone_pcp_init(struct zone *zone)
    >
    > .
    >




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