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    SubjectRe: [PATCH 03/24] io-controller: bfq support of in-class preemption
    Vivek Goyal wrote:
    > o Generally preemption is associated with cross class where if an request
    > from RT class is pending it will preempt the ongoing BE or IDLE class
    > request.
    >
    > o CFQ also does in-class preemtions like a sync request queue preempting the
    > async request queue. In that case it looks like preempting queue gains
    > share and it is not fair.
    >
    > o Implement the similar functionality in bfq so that we can retain the
    > existing CFQ behavior.
    >
    > o This patch creates a bypass path so that a queue can be put at the
    > front of the service tree (add_front, similar to CFQ), so that it will
    > be selected next to run. That's a different thing that in the process
    > this queue gains share.
    >
    > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
    > ---
    > block/elevator-fq.c | 46 +++++++++++++++++++++++++++++++++++++++++-----
    > 1 files changed, 41 insertions(+), 5 deletions(-)
    >
    > diff --git a/block/elevator-fq.c b/block/elevator-fq.c
    > index e5f39cf..f1ab0dc 100644
    > --- a/block/elevator-fq.c
    > +++ b/block/elevator-fq.c
    > @@ -267,7 +267,8 @@ static void bfq_get_entity(struct io_entity *entity)
    > elv_get_ioq(ioq);
    > }
    >
    > -static void bfq_init_entity(struct io_entity *entity, struct io_group *iog)
    > +static inline void
    > +bfq_init_entity(struct io_entity *entity, struct io_group *iog)
    > {
    > entity->sched_data = &iog->sched_data;
    > }
    > @@ -580,7 +581,7 @@ static struct io_entity *bfq_lookup_next_entity(struct io_sched_data *sd,
    > * service received if @entity is active) of the queue to calculate its
    > * timestamps.
    > */
    > -static void __bfq_activate_entity(struct io_entity *entity)
    > +static void __bfq_activate_entity(struct io_entity *entity, int add_front)
    > {
    > struct io_sched_data *sd = entity->sched_data;
    > struct io_service_tree *st = io_entity_service_tree(entity);
    > @@ -625,7 +626,42 @@ static void __bfq_activate_entity(struct io_entity *entity)
    > }
    >
    > st = __bfq_entity_update_prio(st, entity);
    > - bfq_calc_finish(entity, entity->budget);
    > + /*
    > + * This is to emulate cfq like functionality where preemption can
    > + * happen with-in same class, like sync queue preempting async queue
    > + * May be this is not a very good idea from fairness point of view
    > + * as preempting queue gains share. Keeping it for now.
    > + */
    > + if (add_front) {
    > + struct io_entity *next_entity;
    > +
    > + /*
    > + * Determine the entity which will be dispatched next
    > + * Use sd->next_active once hierarchical patch is applied
    > + */
    > + next_entity = bfq_lookup_next_entity(sd, 0);
    > +
    > + if (next_entity && next_entity != entity) {
    > + struct io_service_tree *new_st;
    > + u64 delta;
    > +
    > + new_st = io_entity_service_tree(next_entity);
    > +
    > + /*
    > + * At this point, both entities should belong to
    > + * same service tree as cross service tree preemption
    > + * is automatically taken care by algorithm
    > + */
    > + BUG_ON(new_st != st);

    Hi Vivek,

    I don't quite understand how cross service tree preemption is taken care
    by algorithm, but I've hit this bug while doing some RT I/O and then
    killing it.

    $ ionice -c 1 dd if=/dev/zero of=/tmp/foo bs=1M count=1000 &
    $ killall dd


    ------------[ cut here ]------------
    kernel BUG at block/elevator-fq.c:963!
    invalid opcode: 0000 [#1] SMP
    last sysfs file: /sys/block/sdb/size
    Modules linked in: usb_storage netconsole ipt_REJECT xt_tcpudp iptable_filter ip_tables x_tables bridge stp ipv6 autofs4 hidp rfcomm l2cap bluetooth rfkill sunrpc dm_mirror dm_region_hash dm_log dm_multipath scsi_dh dm_mod video output sbs sbshc battery ac lp snd_hda_codec_analog snd_hda_intel sg dcdbas snd_hda_codec snd_seq_dummy sr_mod snd_seq_oss snd_seq_midi_event snd_seq cdrom snd_seq_device snd_pcm_oss snd_mixer_oss serio_raw snd_pcm snd_timer button parport_pc snd tg3 libphy rtc_cmos i2c_i801 soundcore snd_page_alloc i2c_core pcspkr parport rtc_core rtc_lib ata_piix libata sd_mod scsi_mod ext3 jbd uhci_hcd ohci_hcdehci_hcd [last unloaded: microcode]

    Pid: 5627, comm: crond Not tainted (2.6.31-rc4-io-controller-v7 #62) OptiPlex 745
    EIP: 0060:[<c0535194>] EFLAGS: 00010012 CPU: 0
    EIP is at __bfq_activate_entity+0x1f6/0x370
    EAX: f6af607c EBX: f6af6098 ECX: f6af6070 EDX: f5d1f84c
    ESI: f6af6070 EDI: f5d1f3a8 EBP: f387bcd4 ESP: f387bca4
    DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
    Process crond (pid: 5627, ti=f387b000 task=f38942b0 task.ti=f387b000)
    Stack:
    f6af6070 00000001 f6af6098 00000000 00000001 f6ab4a00 000b3614 00000000
    <0> 00000001 f5d1f3a8 c05323fe f5d1f3a8 f387bce0 c0535cf4 f5d1f3a8 f387bd10
    <0> c05372ff f644fdf4 f71ab368 00000000 f6af6070 00000000 f5d1f84c f6ab4a00
    Call Trace:
    [<c05323fe>] ? cfq_should_preempt+0x0/0xfc
    [<c0535cf4>] ? elv_activate_ioq+0xf/0x27
    [<c05372ff>] ? elv_ioq_request_add+0x2d3/0x30b
    [<c0522b51>] ? elv_insert+0x12c/0x1c0
    [<c0522c74>] ? __elv_add_request+0x8f/0x94
    [<c0528b8d>] ? __make_request+0x303/0x372
    [<c044d23f>] ? mark_held_locks+0x43/0x5b
    [<c052687e>] ? generic_make_request+0x260/0x29c
    [<c0484ca1>] ? mempool_alloc_slab+0xe/0x10
    [<c0484f10>] ? mempool_alloc+0x42/0xe0
    [<c052696f>] ? submit_bio+0xb5/0xbd
    [<c04c5cf1>] ? bio_alloc_bioset+0x25/0xbd
    [<c04c2937>] ? submit_bh+0xdf/0xfc
    [<c04c3fe1>] ? ll_rw_block+0xa4/0xd8
    [<f80bd73c>] ? ext3_bread+0x35/0x5b [ext3]
    [<f80c182f>] ? htree_dirblock_to_tree+0x1f/0x118 [ext3]
    [<f80c198a>] ? ext3_htree_fill_tree+0x62/0x1ac [ext3]
    [<c044d483>] ? trace_hardirqs_on_caller+0xff/0x120
    [<c044d4af>] ? trace_hardirqs_on+0xb/0xd
    [<f80b9a1b>] ? ext3_readdir+0x18f/0x5fe [ext3]
    [<c04b33b5>] ? filldir+0x0/0xb7
    [<c044d483>] ? trace_hardirqs_on_caller+0xff/0x120
    [<c069c632>] ? __mutex_lock_common+0x293/0x2d2
    [<c069c6d4>] ? mutex_lock_killable_nested+0x2e/0x35
    [<c04b3591>] ? vfs_readdir+0x68/0x94
    [<c04b33b5>] ? filldir+0x0/0xb7
    [<c04b36bf>] ? sys_getdents+0x62/0xa1
    [<c04029b4>] ? sysenter_do_call+0x12/0x32
    Code: 00 0f b7 42 4c 8b 4a 44 48 83 f8 02 76 04 0f 0b eb fe 85 c9 75 04 0f 0b eb fe 6b c0 1c 8d 04 01 8945 d0 83 c0 0c 39 45 d8 74 04 <0f> 0b eb fe 8b 5a 10 8b 72 14 8b 47 30 8b 57 34 83 c3 ff 83 d6
    EIP: [<c0535194>] __bfq_activate_entity+0x1f6/0x370 SS:ESP 0068:f387bca4
    ---[ end trace 048abafbc8ce5bf7 ]---

    Regards,
    Jerome

    > + entity->finish = next_entity->finish - 1;
    > + delta = bfq_delta(entity->budget, entity->weight);
    > + entity->start = entity->finish - delta;
    > + if (bfq_gt(entity->start, st->vtime))
    > + entity->start = st->vtime;
    > + }
    > + } else {
    > + bfq_calc_finish(entity, entity->budget);
    > + }
    > bfq_active_insert(st, entity);
    > }
    >
    > @@ -633,9 +669,9 @@ static void __bfq_activate_entity(struct io_entity *entity)
    > * bfq_activate_entity - activate an entity.
    > * @entity: the entity to activate.
    > */
    > -static void bfq_activate_entity(struct io_entity *entity)
    > +static void bfq_activate_entity(struct io_entity *entity, int add_front)
    > {
    > - __bfq_activate_entity(entity);
    > + __bfq_activate_entity(entity, add_front);
    > }
    >
    > /**



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