[dpdk-dev] 回复: [PATCH v5 4/5] lib/bpf: use wait event scheme for Rx/Tx iteration

[Feifei Wang]

> -----邮件原件-----
> 发件人: Feifei Wang
> 发送时间: Wednesday, October 27, 2021 3:04 PM
> 收件人: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> 抄送: dev at dpdk.org; nd <nd at arm.com>; Ruifeng Wang
> <Ruifeng.Wang at arm.com>; nd <nd at arm.com>; nd <nd at arm.com>
> 主题: 回复: [PATCH v5 4/5] lib/bpf: use wait event scheme for Rx/Tx iteration
> 
> 
> 
> > -----邮件原件-----
> > 发件人: dev <dev-bounces at dpdk.org> 代表 Ananyev, Konstantin
> > 发送时间: Tuesday, October 26, 2021 8:57 PM
> > 收件人: Feifei Wang <Feifei.Wang2 at arm.com>
> > 抄送: dev at dpdk.org; nd <nd at arm.com>; Ruifeng Wang
> > <Ruifeng.Wang at arm.com>; nd <nd at arm.com>
> > 主题: Re: [dpdk-dev] [PATCH v5 4/5] lib/bpf: use wait event scheme for
> > Rx/Tx iteration
> >
> >
> > > Hi Feifei,
> > >
> > > > > Instead of polling for cbi->use to be updated, use wait event scheme.
> > > > >
> > > > > Furthermore, delete 'const' for 'bpf_eth_cbi_wait'. This is
> > > > > because of a compilation error:
> > > > > ----------------------------------------------------------------
> > > > > --
> > > > > -----
> > > > > ../lib/eal/include/rte_common.h:36:13: error: read-only variable
> ‘value’
> > > > > used as ‘asm’ output
> > > > >    36 | #define asm __asm__
> > > > >       |             ^~~~~~~
> > > > >
> > > > > ../lib/eal/arm/include/rte_pause_64.h:66:3: note: in expansion
> > > > > of
> > macro ‘asm’
> > > > >    66 |   asm volatile("ldaxr %w[tmp], [%x[addr]]" \
> > > > >       |   ^~~
> > > > >
> > > > > ../lib/eal/arm/include/rte_pause_64.h:96:3: note: in expansion
> > > > > of macro ‘__LOAD_EXC_32’
> > > > >    96 |   __LOAD_EXC_32((src), dst, memorder)     \
> > > > >       |   ^~~~~~~~~~~~~
> > > > >
> > > > > ../lib/eal/arm/include/rte_pause_64.h:167:4: note: in expansion
> > > > > of macro ‘__LOAD_EXC’
> > > > >   167 |    __LOAD_EXC((addr), value, memorder, size) \
> > > > >       |    ^~~~~~~~~~
> > > > >
> > > > > ../lib/bpf/bpf_pkt.c:125:3: note: in expansion of macro
> ‘rte_wait_event’
> > > > >   125 |   rte_wait_event(&cbi->use, UINT32_MAX, ==, puse,
> > > > > ----------------------------------------------------------------
> > > > > --
> > > > > -----
> > > > >
> > > > > Signed-off-by: Feifei Wang <feifei.wang2 at arm.com>
> > > > > Reviewed-by: Ruifeng Wang <ruifeng.wang at arm.com>
> > > > > ---
> > > > >  lib/bpf/bpf_pkt.c | 11 ++++-------
> > > > >  1 file changed, 4 insertions(+), 7 deletions(-)
> > > > >
> > > > > diff --git a/lib/bpf/bpf_pkt.c b/lib/bpf/bpf_pkt.c index
> > > > > 6e8248f0d6..213d44a75a 100644
> > > > > --- a/lib/bpf/bpf_pkt.c
> > > > > +++ b/lib/bpf/bpf_pkt.c
> > > > > @@ -111,9 +111,9 @@ bpf_eth_cbi_unuse(struct bpf_eth_cbi *cbi)
> > > > >   * Waits till datapath finished using given callback.
> > > > >   */
> > > > >  static void
> > > > > -bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi)
> > > > > +bpf_eth_cbi_wait(struct bpf_eth_cbi *cbi)
> > > >
> > > > Hi, Konstantin
> > > >
> > > > For this bpf patch, I delete 'const' through this is contrary to
> > > > what we discussed earlier. This is because if  we keep 'constant'
> > > > here and
> > use 'rte_wait_event'
> > > > new macro, compiler will report error. And earlier the arm version
> > > > cannot be compiled due to I forgot enable "wfe" config in the
> > > > meson file,
> > so this issue can not happen before.
> > >
> > >
> > > Honestly, I don't understand why we have to remove perfectly valid
> 'const'
> > qualifier here.
> > > If this macro can't be used with pointers to const (still don't
> > > understand why), then let's just not use this macro here.
> > > Strictly speaking I don't see much benefit here from it.
> > >
> > > >
> > > > >  {
> > > > > -	uint32_t nuse, puse;
> > > > > +	uint32_t puse;
> > > > >
> > > > >  	/* make sure all previous loads and stores are completed */
> > > > >  	rte_smp_mb();
> > > > > @@ -122,11 +122,8 @@ bpf_eth_cbi_wait(const struct bpf_eth_cbi
> > > > > *cbi)
> > > > >
> > > > >  	/* in use, busy wait till current RX/TX iteration is finished */
> > > > >  	if ((puse & BPF_ETH_CBI_INUSE) != 0) {
> > > > > -		do {
> > > > > -			rte_pause();
> > > > > -			rte_compiler_barrier();
> > > > > -			nuse = cbi->use;
> > > > > -		} while (nuse == puse);
> > > > > +		rte_wait_event(&cbi->use, UINT32_MAX, ==, puse,
> > > > > +				__ATOMIC_RELAXED);
> >
> > After another thought, if we do type conversion at macro invocation time:
> >
> > bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi) {
> >   ...
> >   rte_wait_event((uint32_t *)&cbi->use, UINT32_MAX, ==, puse,
> > __ATOMIC_RELAXED);
> >
> > would that help?
> 
> I try to with this and it will report compiler warning:
> ' cast discards ‘const’ qualifier'.
> I think this is due to that in rte_wait_event macro, we use
> typeof(*(addr)) value = 0;
>  and value is defined as "const uint32_t", but it should be able to be updated.
> 

Correct a little.
The explain is for 'asm error' in the commit message.

> Furthermore, this reflects the limitations of the new macro, it cannot be
> applied when 'addr' is type of 'const'. Finally, I think I should give up the
> change for "bpf".
> >
> >
> > > > >  	}
> > > > >  }
> > > > >
> > > > > --
> > > > > 2.25.1